Very fast Erlang NIF encoder/decoder for JSON, YAML, and CSV,
built around hand-rolled recursive-descent decoders and direct
term-to-text encoders that produce/consume native Erlang terms in a
single pass. The JSON implementation was inspired by the
glaze C++ library; glazer has
since matured into a standalone implementation with no external C++
dependencies, and extended the same approach to YAML and CSV, with
performance and features unmatched by other existing libraries for these
formats.
## Performance
- **[JSON](#performance-1)**: faster than every other library benchmarked on
both encoding and decoding — consistently ~25–40% ahead of `torque`
(Rust `sonic-rs` NIF), and well ahead of `simdjsone`, `jiffy`, and the
pure-Elixir libraries `jason`, `thoas`, `euneus`, and OTP's built-in `json`.
- **[YAML](#benchmarking-yaml)**: 2–7× faster than `yaml_rustler` and
`fast_yaml`, and ~25–75× faster than the pure-Erlang `yamerl`/`ymlr`.
- **[CSV](#benchmarking-csv)**: 4–12× faster than `nimble_csv`, and tens to
hundreds of times faster than `csv` and `erl_csv` (which time out on
large inputs).
Each chart compares glazer against other libraries for JSON/YAML/CSV
decode and encode on a representative small/medium/large file. Charts are
generated from the tables below via `scripts/gen_bench_charts.py`.
Benchmark tables:
- [Benchmarking JSON](#benchmarking-json)
- [Benchmarking YAML](#benchmarking-yaml)
- [Benchmarking CSV](#benchmarking-csv)
## Features
### JSON
- Decoding straight to Erlang terms: maps, lists, binaries, integers
(including bignums), floats, booleans, and `null`
- Encoding Erlang terms straight to JSON, including big integers
- Incremental/streaming decoding of partial input (e.g. NDJSON over a
socket) via `stream_decoder/0,1`, `stream_feed/2`, `stream_eof/1`
- Configurable representation of JSON `null` and JSON object keys
- `minify/1` and `prettify/1` helpers
- Standalone big-integer encode/decode helpers
(`encode_integer/1`, `decode_integer/1`, `try_decode_integer/1`)
- `query/2,3`: run a [jq](https://jqlang.org/) filter over a JSON
document, returning decoded Erlang terms (requires `glazer` to be built
with `libjq` available — see [jq filter support](#jq-filter-support))
- `glazer:find/2` and `glazer:compile_path/1`: look up value(s) in a
decoded term using a small subset of jq path syntax (`.a.b[].c[0]`),
with no `libjq` dependency
### YAML
- Decoding YAML mappings/sequences/scalars to Erlang maps/lists/scalars,
including big integers
- Encoding Erlang terms to YAML in block style
- Configurable representation of YAML `null` and mapping keys, with
optional YAML 1.1 boolean compatibility (`yes`/`no`/`on`/`off`)
### CSV
- RFC 4180 CSV encoding/decoding via `decode/1,2` and `encode/1,2`,
with optional header-row support
- Incremental/streaming CSV decoding via `stream_decoder/0,1`,
`stream_feed/2`, `stream_eof/1`
## Scope
`glazer` targets formats that map naturally onto a tree of Erlang
maps/lists/scalars — JSON and YAML both fit this model directly, so a
single decode/encode pair can convert losslessly between the format and
native terms. XML is intentionally **not** planned: its data model
(tagged elements, attributes, mixed text/element content, namespaces,
processing instructions, entities) has no single natural Erlang term
representation, and any choice (xmerl-style tuples, JSON-like maps with
`@attr`/`#text` keys, etc.) is a lossy or awkward fit compared to formats
that are already trees of scalars and collections. Erlang's standard
library already ships `xmerl` for XML; there's little value in
duplicating it here with a different, opinionated term shape.
## Installation
**Erlang (`rebar.config`)**:
```erlang
{deps, [
{glazer, "~> 0.5"}
]}.
```
**Elixir (`mix.exs`)**:
```elixir
def deps do
[
{:glazer, "~> 0.5"}
]
end
```
### Building
Building the NIF requires a C++23 compiler (GCC 12+ or Clang 16+) and
`make`. There are no external C++ library dependencies — all C++ code is
self-contained in `c_src/`. A plain
```sh
make
```
builds `priv/glazer.so` and compiles the Erlang sources. For the fastest
performance, run a Profile-Guided Optimisation (PGO) build instead:
```sh
make optimize
```
or
```sh
OPTIMIZE=1 make
```
This performs three steps automatically: compiles an instrumented binary,
runs the test suite to collect real branch-frequency data, then recompiles
with those profiles applied. The resulting `.so` typically outperforms a
plain `-O3` build by 5–15% on realistic JSON workloads.
`glazer` is an Erlang application with a Rebar-based C++ NIF build;
`mix` invokes the same top-level `Makefile`/`rebar3 compile` path
described above, so the same C++23 compiler requirement applies.
Once compiled, call it via the `:glazer` module from Elixir:
**Erlang:**
```erlang
1> glazer_json:decode(~"{\"a\":1,\"b\":[true,null,3.5]}")
#{<<"a">> => 1,<<"b">> => [true,null,3.5]}
```
**Elixir:**
```elixir
iex> :glazer_json.encode(%{"a" => 1, "b" => [true, :null, 3.5]})
"{\"a\":1,\"b\":[true,null,3.5]}"
```
Use the `use_nil`/`{null_term, nil}` option (see
[Null term configuration](#null-term-configuration) below) to get idiomatic
Elixir `nil` instead of the atom `:null`.
## JSON
### Usage
```erlang
1> glazer_json:decode(<<"{\"a\":1,\"b\":[true,null,3.5]}">>).
#{<<"a">> => 1, <<"b">> => [true, null, 3.5]}
2> glazer_json:encode(#{<<"a">> => 1, <<"b">> => [true, null, 3.5]}).
<<"{\"a\":1,\"b\":[true,null,3.5]}">>
3> glazer_json:encode(#{a => 1}, [pretty]).
<<"{\n \"a\": 1\n}">>
4> glazer_json:minify(<<" { \"a\" : 1 } ">>).
{ok, <<"{\"a\":1}">>}
5> glazer_json:prettify(<<"{\"a\":1}">>).
{ok, <<"{\n \"a\": 1\n}">>}
```
### Streaming
For input that arrives in chunks — e.g. reading a large document
incrementally, or consuming newline-delimited JSON (NDJSON) from a
socket or file — `stream_decoder/0,1` provides a small stateful
wrapper that buffers partial input and decodes each JSON value as soon
as it's complete, without re-parsing bytes you've already seen:
```erlang
1> D0 = glazer_json:stream_decoder(),
2> {Vals1, D1} = glazer_json:stream_feed(D0, <<"{\"a\":1} {\"b\":">>),
3> Vals1.
[#{<<"a">> => 1}]
4> {Vals2, D2} = glazer_json:stream_feed(D1, <<"2}">>),
5> Vals2.
[#{<<"b">> => 2}]
6> glazer_json:stream_eof(D2).
{ok, []}
```
`stream_feed/2` returns the list of values completed by the chunk just
fed (possibly empty, possibly more than one if the chunk completes
several values) along with the updated decoder state to pass to the
next call. Once the input is exhausted, call `stream_eof/1` to flush
any trailing bare scalar (numbers, strings, etc. have no closing
delimiter of their own) and surface an error if the buffer holds an
incomplete value:
```erlang
1> D0 = glazer_json:stream_decoder(),
2> {[], D1} = glazer_json:stream_feed(D0, <<" 42">>),
3> glazer_json:stream_eof(D1).
{ok, [42]}
```
`stream_decoder/1` accepts the same options as `decode/2` (e.g.
`{keys, atom}`, `use_nil`) and applies them to every decoded value.
A typical read loop calls `stream_feed/2` for each chunk while more data
may still arrive, and `stream_eof/1` once the socket closes to flush any
trailing value:
```erlang
loop(Socket, D0) ->
case gen_tcp:recv(Socket, 0) of
{ok, Chunk} ->
{Vals, D1} = glazer_json:stream_feed(D0, Chunk),
handle_values(Vals),
loop(Socket, D1);
{error, closed} ->
case glazer_json:stream_eof(D0) of
{ok, Trailing} -> handle_values(Trailing);
{error, Reason} -> handle_truncated_stream(Reason)
end
end.
```
#### Efficiency
`stream_feed/2` only scans for value *boundaries* incrementally —
the scanner carries a small resumable cursor (`scan_state()`) that
remembers how far it has already looked (nesting depth, whether it's
inside a string, escape state, …), so each call to `scan/2` resumes
from where the previous one left off rather than re-walking the whole
buffer from byte zero. Once a complete value's end offset is known,
that slice is decoded exactly once via the same NIF-backed decoder
used by `decode/2` — there's no intermediate tokenization or tree
representation, and no byte is ever scanned or decoded twice. The only
buffering cost is concatenating newly-arrived chunks onto the
not-yet-complete tail of the input.
This makes `stream_feed/2` well suited to byte-at-a-time or
small-chunk feeding (e.g. consuming a `gen_tcp`/`gen_statem` socket
buffer as it fills) without the quadratic-rescan cost a naive
"concatenate and retry full decode" loop would incur on large or
slow-arriving documents.
Under the hood, `stream_feed/2` is built on `scan/1,2` — a low-level
primitive that scans a buffer for the byte offset where the next JSON
value ends (or reports that more input is needed) without doing a full
decode. It's exposed directly for callers that want to implement their
own framing/buffering strategy:
```erlang
1> glazer_json:scan(<<"{\"a\":1} {\"b\":2}">>).
{complete, 7}
2> glazer_json:scan(<<"{\"a\":">>).
{incomplete, ScanState}
3> glazer_json:scan(<<"{\"a\":1}">>, ScanState).
{complete, 7}
```
`stream_decoder/0,1`, `stream_feed/2`, `stream_eof/1` and
`scan/1,2` are JSON-only — see [YAML streaming](#streaming-1) and
[CSV streaming](#streaming-2) below for the other formats.
### Null term configuration
By default, JSON/YAML `null` decodes to (and `null` encodes from) the atom
`null`, and this same atom is used as the default null term throughout the
library (e.g. for the CSV `on_failure => null` field option). This can be
overridden:
- Application-wide, via the `null` environment key — set this once in
the application's config and every call uses it as the default:
**Erlang** (`rebar.config`):
```erlang
{glazer, [{null, nil}]}
```
**Elixir** (`config.exs`):
```erlang
config :glazer, null: nil
```
- Per call, with the `use_nil` shorthand or the `{null_term, Atom}`
option (see [Decode options](#decode-options-glazer_jsondecode2) below).
Per-call options always take precedence over the application-wide
default.
### JSON decode options
| Option | Description |
|---|---|
| `object_as_tuple` | Decode JSON objects as `{[{Key, Value}]}` proplist tuples (jiffy-style) instead of maps (default) |
| `use_nil` | Use the atom `nil` for JSON `null` |
| `{null_term, Atom}` | Use `Atom` for JSON `null` |
| `{keys, atom}` | Decode object keys as atoms (via `binary_to_atom/2`-equivalent) |
| `{keys, existing_atom}` | Decode object keys as existing atoms, falling back to binaries for unknown atoms |
| `{keys, binary}` | Decode object keys as binaries (default) |
| `dedupe_keys` | With `object_as_tuple`, eliminate duplicate object keys, keeping the last occurrence's value (and position) |
```erlang
1> glazer_json:decode(<<"{\"a\":1}">>, [object_as_tuple]).
{[{<<"a">>, 1}]}
2> glazer_json:decode(<<"{\"a\":1}">>, [{keys, atom}]).
#{a => 1}
3> glazer_json:decode(<<"null">>, [use_nil]).
nil
4> glazer_json:decode(<<"null">>, [{null_term, undefined}]).
undefined
5> glazer_json:decode(<<"{\"a\":1,\"a\":2}">>).
#{<<"a">> => 2}
6> glazer_json:decode(<<"{\"a\":1,\"a\":2}">>, [object_as_tuple]).
{[{<<"a">>, 1}, {<<"a">>, 2}]}
7> glazer_json:decode(<<"{\"a\":1,\"a\":2}">>, [object_as_tuple, dedupe_keys]).
{[{<<"a">>, 2}]}
```
> [!NOTE]
> A JSON object with duplicate keys cannot be represented as an Erlang map,
> so decoding to maps (the default) and `{keys, atom | existing_atom}` always
> dedupe duplicate keys, last value wins, regardless of `dedupe_keys`. With
> `object_as_tuple`, duplicate keys are preserved as-is unless `dedupe_keys`
> is given.
### jq filter support
If [`libjq`](https://jqlang.org/) and its headers (`jq.h`/`jv.h`) are
available when `glazer` is built, `query/2,3` runs a jq filter
program against a JSON document and returns one Erlang term per value
produced by the filter (decoded using the same options as
`decode/2`):
```erlang
1> glazer_json:query(<<"{\"a\":[1,2,3]}">>, <<".a[]">>).
{ok, [1, 2, 3]}
2> glazer_json:query(<<"{\"a\":1}">>, <<".b">>).
{ok, [null]}
3> glazer_json:query(<<"{\"a\":{\"b\":2}}">>, <<".">>, [{keys, atom}]).
{ok, [#{a => #{b => 2}}]}
4> glazer_json:query(<<"not json">>, <<".">>).
{error, invalid_input}
5> glazer_json:query(<<"{\"a\":1}">>, <<"bad syntax (((">>).
{error, jq_decode_error}
```
If `libjq` was not available at build time, `query/2,3` returns
`{error, jq_not_available}`. Build detection is automatic — `make` probes
for `jq.h`/`libjq` and only enables this feature if found, so `glazer`
still builds and works without `libjq` installed.
### API
All functions below are in `glazer_json`.
| Function | Description |
|---|---|
| `decode/1`, `decode/2` | Decode a JSON binary or iolist to an Erlang term |
| `try_decode/1`, `try_decode/2` | Decode a JSON binary or iolist, returning `{ok, Term}` or `{error, {parse_error, Msg}}` instead of raising |
| `encode/1`, `encode/2` | Encode an Erlang term to a JSON binary |
| `minify/1` | Remove unnecessary whitespace from a JSON document |
| `prettify/1` | Pretty-print a JSON document with two-space indentation |
| `read_file/1`, `read_file/2` | Read a file and decode its contents as JSON |
| `write_file/2`, `write_file/3` | Encode a term to JSON and write it to a file |
| `scan/1`, `scan/2` | Scan a buffer for the end offset of the next complete JSON value |
| `stream_decoder/0`, `stream_decoder/1` | Create an incremental-decode state for chunked input |
| `stream_feed/2` | Feed a chunk to a stream decoder, returning completed values |
| `stream_eof/1` | Flush a stream decoder at end-of-input |
| `query/2`, `query/3` | Run a [jq](https://jqlang.org/) filter over a JSON document, returning `{ok, [Term]}` (requires `libjq`) |
### Benchmarking JSON
A comparison benchmark against other JSON libraries (`simdjsone`,
`jiffy`, `jason`, `thoas`, `euneus`, OTP's built-in `json`, and
`torque`) is available via:
```sh
$ PARALLEL=2 make bench-json
==> Running benchmarks with parallelism: 2
(numbers in µs)
JSON twitter (616.7K) twitter2 (758.0K) openrtb (1.2K) esad (1.3K) small (0.1K)
decode encode decode encode decode encode decode encode decode encode
-------------------------------------------------------------------------------------------------------------
glazer 4379.2 1143.4 5132.9 2586.7 7.5 8.7 6.7 4.0 1.2 1.0
torque 6089.2 1643.8 8087.6 3091.0 10.7 9.8 9.3 6.2 1.7 1.3
simdjsone 5847.3 5019.7 8719.8 8620.6 14.4 17.7 12.1 12.6 1.9 3.6
jiffy 7868.6 3615.6 9779.9 6532.6 16.8 15.2 12.4 9.1 2.5 3.8
jason 13509.0 11248.6 25267.6 20837.6 33.5 30.0 19.7 25.0 4.4 2.9
thoas 13679.7 12466.1 25638.7 22607.2 31.2 33.0 25.1 29.9 3.2 3.9
euneus 14699.8 10247.2 18646.5 16886.6 29.1 25.2 16.7 14.6 4.0 4.6
json 14315.5 9718.9 17844.3 16473.5 28.3 25.3 19.2 12.3 4.0 4.5
```
(requires the `bench`/`dev` Mix dependencies — see `mix.exs`).
## YAML
### Usage
`decode/1,2` decodes a YAML document to an Erlang term — mappings
become maps, sequences become lists, and scalars become the matching
Erlang type (binaries, numbers, booleans, or `null`):
```erlang
1> glazer_yaml:decode(<<"a: 1\nb:\n - true\n - null\n - 3.5\n">>).
#{<<"a">> => 1, <<"b">> => [true, null, 3.5]}
2> glazer_yaml:encode(#{<<"a">> => 1, <<"b">> => [true, null, 3.5]}).
<<"a: 1\nb:\n - true\n - null\n - 3.5\n">>
```
`encode/1,2` encodes an Erlang term to YAML in block style
(2-space indentation, sequences at the same indentation as the mapping
key that owns them).
### Streaming
There is no incremental YAML decoder. YAML's block styles have no
closing delimiter — a mapping or sequence simply ends at a dedent or
end-of-input — so there is no way to scan a partial buffer for "is this
value complete yet?" the way [`scan/1,2`](#efficiency) does for
JSON's bracket-balanced syntax. Decode full YAML documents with
`decode/1,2` once they are fully buffered.
### YAML decode options
| Option | Description |
|---|---|
| `use_nil` | Use the atom `nil` for YAML `null`/`~`/empty values |
| `{null_term, Atom}` | Use `Atom` for YAML `null`/`~`/empty values |
| `{keys, atom}` | Decode mapping keys as atoms |
| `{keys, existing_atom}` | Decode mapping keys as existing atoms, falling back to binaries for unknown atoms |
| `{keys, binary}` | Decode mapping keys as binaries (default) |
| `yaml_1_1_bools` | Additionally treat `yes`/`no`/`on`/`off` (and case variants) as booleans, per the YAML 1.1 core schema. By default (YAML 1.2 core schema) only `true`/`false` are recognized as booleans |
```erlang
1> glazer_yaml:decode(<<"a: ~\n">>, [use_nil]).
#{<<"a">> => nil}
2> glazer_yaml:decode(<<"a: 1\n">>, [{keys, atom}]).
#{a => 1}
3> glazer_yaml:decode(<<"a: yes\n">>, [yaml_1_1_bools]).
#{<<"a">> => true}
```
### YAML encode options
| Option | Description |
|---|---|
| `use_nil` | Treat the atom `nil` as YAML `null` |
| `{null_term, Atom}` | Treat `Atom` as YAML `null` |
```erlang
1> glazer_yaml:encode(#{<<"a">> => nil}, [use_nil]).
<<"a: null\n">>
```
### API
All functions below are in `glazer_yaml`.
| Function | Description |
|---|---|
| `decode/1`, `decode/2` | Decode a YAML binary or iolist to an Erlang term |
| `try_decode/1`, `try_decode/2` | Decode YAML, returning `{ok, Term}` or `{error, Msg}` instead of raising |
| `encode/1`, `encode/2` | Encode an Erlang term to a YAML binary in block style |
| `read_file/1`, `read_file/2` | Read a file and decode its contents as YAML |
| `write_file/2`, `write_file/3` | Encode a term to YAML and write it to a file |
### Benchmarking YAML
```sh
$ PARALLEL=2 make bench-yaml
==> Running benchmarks with parallelism: 2
(numbers in µs)
YAML openrtb (1.3K) esad (1.3K) small (0.1K)
decode encode decode encode decode encode
-------------------------------------------------------------------------
glazer 81.0 14.7 19.9 7.9 11.5 2.2
yaml_rustler 195.3 n/a 103.9 n/a 16.9 n/a
fast_yaml 254.9 69.5 141.4 54.4 26.7 7.6
yamerl 2014.4 n/a 1486.2 n/a 676.1 n/a
ymlr n/a 62.6 n/a 46.1 n/a 5.9
```
## CSV
### Usage
`decode/1,2` decodes an RFC 4180 CSV document to `#{headers => nil|[...],
data => Rows}`, where `Rows` is a list of rows, each row a list of binary
fields by default:
```erlang
1> glazer_csv:decode(<<"name,age\nAlice,30\nBob,25\n">>).
#{headers => nil,
data => [[<<"name">>,<<"age">>],[<<"Alice">>,<<"30">>],[<<"Bob">>,<<"25">>]]}
2> glazer_csv:encode([[<<"name">>, <<"age">>], [<<"Alice">>, 30]]).
<<"name,age\r\nAlice,30\r\n">>
```
With the `headers` option, the first row is captured as column names in
`headers` and each subsequent row decodes to a map when combined with
`{return, map}`; `encode/2` with `headers` does the reverse, deriving the
header row from the first map's keys:
```erlang
1> glazer_csv:decode(<<"name,age\nAlice,30\n">>, [headers, {return, map}]).
#{headers => [<<"name">>,<<"age">>],
data => [#{<<"name">> => <<"Alice">>, <<"age">> => <<"30">>}]}
2> glazer_csv:encode([#{<<"name">> => <<"Alice">>, <<"age">> => 30}], [headers]).
<<"name,age\r\nAlice,30\r\n">>
```
Fields containing the delimiter, a double quote, or a line break are
quoted automatically on encode (with embedded quotes doubled), and
unquoted on decode. The delimiter defaults to `,` and can be changed via
`{delimiter, Char}`; the encoded line ending defaults to `\r\n` per
RFC 4180 and can be changed to `\n` via `{line_ending, lf}`.
### Streaming
For input that arrives in chunks, `stream_decoder/0,1` provides the
same kind of stateful wrapper as [JSON streaming](#streaming): it buffers
partial input and decodes each row as soon as its terminating line break
is seen, via `decode/2` on that single row. A small scanner tracks
whether the cursor is inside a quoted field across chunks, so a `\n`/`\r\n`
inside a quoted field doesn't end the row:
```erlang
1> D0 = glazer_csv:stream_decoder(),
2> {Rows1, D1} = glazer_csv:stream_feed(D0, <<"a,b\n1,2\n3,">>),
3> Rows1.
[[<<"a">>,<<"b">>],[<<"1">>,<<"2">>]]
4> {Rows2, D2} = glazer_csv:stream_feed(D1, <<"4\n">>),
5> Rows2.
[[<<"3">>,<<"4">>]]
6> glazer_csv:stream_eof(D2).
{ok, []}
```
`stream_feed/2` returns the rows completed by the chunk just fed
(possibly empty, possibly more than one) along with the updated decoder
state. Once the input is exhausted, call `stream_eof/1` to flush a
trailing row that has no terminating line break, or surface an error if
the buffered bytes don't form a valid row:
```erlang
1> D0 = glazer_csv:stream_decoder(),
2> {Rows1, D1} = glazer_csv:stream_feed(D0, <<"a,b\n1,2">>),
3> Rows1.
[[<<"a">>,<<"b">>]]
4> glazer_csv:stream_eof(D1).
{ok, [[<<"1">>,<<"2">>]]}
```
`stream_decoder/1` accepts the same options as `decode/2`. With
the `headers` option, the first complete row is captured as the header and
used to decode every subsequent row (as a map when combined with
`{return, map}`); no row is emitted for the header itself. Blank lines are
skipped, matching `decode/2`.
### CSV decode options
| Option | Description |
|---|---|
| `{delimiter, Char}` | Field delimiter (default `$,`) |
| `headers` | Treat the first row as column names (shorthand for `{headers, binary}`) |
| `{headers, [Name, ...]}` | Use the given list of atoms or binaries as column names; the first data row is **not** consumed as a header |
| `{headers, binary}` | First row is binary column names (same as bare `headers`) |
| `{headers, string}` | Alias for `{headers, binary}` |
| `{headers, atom}` | First row → atom column names (via `binary_to_atom/2`-equivalent) |
| `{headers, existing_atom}` | First row → existing-atom column names, falling back to binaries for unknown atoms |
| `{headers, charlist}` | First row → column names as lists of Unicode codepoints |
| `{return, list}` | Data rows are lists of field values (default) |
| `{return, tuple}` | Data rows are tuples of field values |
| `{return, map}` | Data rows are maps keyed by column names; requires `headers` or `{headers, ...}`. Raises `duplicate_header` on duplicate column names |
| `{fields, Specs}` | Convert each column's field from a binary, positionally — see [Field type conversion](#field-type-conversion) |
| `{skip, N}` | Skip the first `N` data rows (after any header row) |
| `{skip, {From, To}}` | Process only data rows `From..To` (1-based inclusive); equivalent to `{skip, From-1}` plus `{limit, To-From+1}` |
| `{limit, N}` | Process at most `N` data rows (after skipping) |
| `{null_term, Atom}` | Use `Atom` as the value produced by `on_failure => null` (default `null`) |
### Field type conversion
The `{fields, Specs}` decode option converts each column's field from a
binary to the given Erlang type. `Specs` is a list applied positionally —
the Nth spec applies to the Nth column, regardless of whether `headers` is
set. Columns beyond the end of `Specs` are left as binaries.
```erlang
1> glazer_csv:decode(<<"name,age,active,joined\nAlice,30,true,2024-01-15T10:30:00Z\n">>,
.. [headers, {fields, [binary, integer, boolean,
.. {datetime, <<"%Y-%m-%dT%H:%M:%SZ">>}]}]).
[#{<<"name">> => <<"Alice">>, <<"age">> => 30, <<"active">> => true,
<<"joined">> => 1705314600}]
```
Each element of `Specs` is either a `Type` directly, or a map
`#{type => Type, default => Term, on_failure => OnFailure}` for more
control (see below). `Type` is one of:
| Type | Description |
|---|---|
| `integer` | Parse the field as an integer |
| `{float, Precision}` | Parse the field as a float, rounded to `Precision` decimal digits |
| `boolean` | Parse `"true"`/`"false"` (any case) as `true`/`false` |
| `{datetime, InputFormat}` | Parse with a `strptime`-like format string and convert to Unix epoch seconds (UTC) |
| `binary` | Leave the field as a binary (default) |
| `charlist` | Convert the field to a list of Unicode code points |
| `existing_atom` | Convert to an existing atom, falling back to a binary if no such atom exists |
| `{atom, ExistingAtoms}` | Convert to an atom only if the field's text matches (and exists as) one of `ExistingAtoms`, falling back to a binary otherwise |
`InputFormat` supports the directives `%Y %y %m %d %H %M %S %f %z` (and
`%%` for a literal `%`); any other character must match the input
literally, and a space matches a run of one-or-more whitespace characters.
`%z` accepts `Z`, `+HHMM`, or `+HH:MM`-style offsets; fractional seconds
(`%f`) are parsed but discarded. The result is always in UTC.
#### `default` and `on_failure`
Using the map form `#{type => Type, default => Term, on_failure => OnFailure}`:
- `default` (when given) is used in place of the converted value whenever
the raw CSV field is empty.
- `on_failure` controls what happens when a *non-empty* field fails to
convert to `Type` (default `binary`):
| `on_failure` | Behavior |
|---|---|
| `binary` | Leave the field as the original binary (default) |
| `raise` | Raise `{invalid_field_value, Row, Column}` (1-based), or return `{error, Reason}` from `try_decode/2` |
| `default` | Use the spec's `default` value (falls back to `binary` if no `default` is given) |
| `null` | Use the configured null term: `{null_term, Atom}` if given, otherwise the library-wide null term (see [Null term configuration](#null-term-configuration) and `{null_term, Atom}` below) |
```erlang
1> glazer_csv:decode(<<"1\nbad\n">>,
.. [{fields, [#{type => integer, on_failure => raise}]}]).
** exception error: {invalid_field_value,2,1}
2> glazer_csv:decode(<<"1\nbad\n">>,
.. [{fields, [#{type => integer, default => 0, on_failure => default}]}]).
[[1],[0]]
3> glazer_csv:decode(<<"1\nbad\n">>,
.. [{null_term, nil},
.. {fields, [#{type => integer, on_failure => null}]}]).
[[1],[nil]]
```
`{null_term, Atom}` only affects `on_failure => null` for that call. Without
it, `on_failure => null` falls back to the library-wide null term — `null`
by default, or whatever atom is configured via the
[Null term configuration](#null-term-configuration)
application env var (`{glazer, [{null, Atom}]}`).
### CSV Encode options
| Option | Description |
|---|---|
| `{delimiter, Char}` | Field delimiter (default `$,`) |
| `headers` | Input is a list of maps; the first map's keys become the header row, and subsequent maps are encoded as rows in that column order (missing keys produce empty fields) |
| `{headers, [Name, ...]}` | Input is a list of maps; uses the given list of atoms or binaries (matching the maps' key type) as the column order and header row, instead of deriving it from the first map's keys (missing keys produce empty fields) |
| `{line_ending, lf \| crlf}` | Line terminator (default `crlf`, per RFC 4180) |
### API
All functions below are in `glazer_csv`.
| Function | Description |
|---|---|
| `decode/1`, `decode/2` | Decode a CSV binary or iolist to a list of rows (or maps with `headers`) |
| `try_decode/1`, `try_decode/2` | Decode CSV, returning `{ok, Rows}` or `{error, Reason}` instead of raising |
| `encode/1`, `encode/2` | Encode a list of rows (or maps with `headers`) to a CSV binary |
| `read_file/1`, `read_file/2` | Read a file and decode its contents as CSV |
| `write_file/2`, `write_file/3` | Encode rows to CSV and write them to a file |
| `stream_decoder/0`, `stream_decoder/1` | Create an incremental CSV decode state for chunked input |
| `stream_feed/2` | Feed a chunk to a CSV stream decoder, returning completed rows |
| `stream_eof/1` | Flush a CSV stream decoder at end-of-input |
### Benchmarking CSV
```sh
$ PARALLEL=2 make bench-csv
==> Running benchmarks with parallelism: 2
(numbers in µs)
CSV small (1.3K) medium (130.9K) large (3433.1K)
decode encode decode encode decode encode
-----------------------------------------------------------------------------------
glazer 10.7 3.3 1289.6 469.5 42617.2 16240.1
nimble_csv 44.8 38.8 4582.9 3204.4 238366.4 120585.9
csv 99.3 257.3 8335.2 24393.9 TIMEOUT TIMEOUT
erl_csv 705.5 427.4 54950.5 34607.9 TIMEOUT TIMEOUT
```
## Big integers
JSON/YAML/CSV numbers that don't fit into a 64-bit integer are decoded as
Erlang big integers (and big integers are encoded back to their exact
decimal representation).
### API
| Function | Description |
|---|---|
| `encode_integer/1` | Encode an integer to its JSON decimal-string representation |
| `decode_integer/1` | Decode a JSON number string to an Erlang integer, raising on invalid input |
| `try_decode_integer/1` | Decode a JSON number string to an Erlang integer, returning `{ok, Int}` or `{error, invalid_number_format}` |
`encode_integer/1` and `decode_integer/1`/`try_decode_integer/1` expose the
same conversion routines directly, independent of JSON/YAML/CSV parsing/encoding:
```erlang
1> glazer:encode_integer(123456789012345678901234567890).
<<"123456789012345678901234567890">>
2> glazer:decode_integer(<<"123456789012345678901234567890">>).
123456789012345678901234567890
3> glazer:try_decode_integer(<<"not a number">>).
{error, invalid_number_format}
```
See the module's documentation (`src/glazer.erl`) for full type
specs and details.
## Limitations
### Nesting depth
The JSON and YAML decoders both cap recursion at **256 levels** of nesting
(arrays/objects for JSON; mappings/sequences for YAML). Inputs that exceed
this limit are rejected with a decode error rather than crashing the VM by
overflowing the C stack.
| Format | Limit | Error returned |
|--------|-------|----------------|
| JSON | 256 | `{error, <<"exceeded maximum nesting depth at offset N">>}` |
| YAML | 256 | `{error, <<"exceeded maximum nesting depth at offset N">>}` |
256 levels is sufficient for any reasonable real-world document; it is
deliberately not configurable, because the limit exists to protect the
Erlang VM process (the NIF runs on the scheduler thread) from runaway
recursive descent on adversarial input.
## Testing
```sh
make test
```
runs the EUnit test suite via `rebar3 eunit`.
## Performance Optimization Details
`glazer` is faster than all competitors on both encoding and decoding in all
data formats - JSON/YAML/CSV. On JSON decoding it leads `torque` (Rust
`sonic-rs` NIF) by ~25–40% across every benchmarked workload, and on encoding
by ~10–30%. Both sit well ahead of the remaining contenders (`simdjsone`,
`jiffy`, and the pure-Elixir libraries `jason`, `thoas`, `euneus`, and OTP's
built-in `json`).
- **No tuple-of-binaries intermediate representation.** `glazer` decodes
straight to native Erlang terms (maps, lists, binaries, numbers) and
encodes straight from them, in a single pass, with no generic JSON-tree
staging step — minimizing allocation and copying on both the decode and
encode paths.
- **Big integer support.** numbers that overflow 64 bits decode to
Erlang bignums (and encode back to their exact decimal form) — see
[Big integers](#big-integers).
- **No external C++ dependencies.** The NIF is fully self-contained —
no CMake, no vendored third-party library to pull at build time, so it's
easier to use as a dependency since it doesn't have reliance on other
toolchains such as `sonic-rs` by other libraries that use Rust.
A few implementation techniques in `c_src/glazer_nif.cpp` account for most
of the gap over the slower contenders:
- **Single-pass, zero-copy decode/encode.** As noted above, there's no
intermediate generic JSON tree — the decoder builds Erlang terms directly
from the input bytes (string keys/values are views into the original
binary whenever no escaping is needed) and the encoder writes JSON bytes
directly from Erlang terms. This removes a whole staging
allocate-and-copy pass that tree-based decoders pay for.
- **Inline, growable output buffer (`OutBuf`).** Encoding writes into a
4 KB stack-allocated buffer first; only documents that exceed that spill
to the heap, growing geometrically via `malloc`/`realloc` (the latter
resizes in place when possible, avoiding a copy on every growth — a
plain `new[]`/`delete[]` doubling strategy can't do this).
- **Key cache for repeated object keys (`KeyCache`).** Real-world JSON
documents reuse the same small set of key strings heavily (e.g. a
Twitter feed has ~13K key occurrences across only ~94 distinct keys).
`KeyCache` is an open-addressed hash table (power-of-two size, linear
probing, FNV-1a hash with a precomputed-hash fast-reject before the
`memcmp`) that lets a repeated key reuse the same already-built
`ERL_NIF_TERM` binary instead of paying `enif_make_new_binary` + `memcpy`
again. It's only engaged for inputs above a size threshold
(`KEY_CACHE_MIN_SIZE`), since small payloads (RPC-sized messages) rarely
repeat keys enough to amortize the lookup cost.
- **Epoch-counter lazy clearing.** Both `KeyCache` and the scratch buffers
it touches need to start "empty" on every decode call, but
zero-initializing a multi-KB table for every single call — including
tiny documents that never populate it — would cost more than the cache
saves. Instead each cache entry carries a generation/`epoch` tag; a slot
is considered live only if its `epoch` matches the cache's current
`m_epoch` (itself seeded from a process-wide monotonically-increasing
counter, so leftover garbage from a prior stack frame can never
coincidentally look live). This makes cache construction effectively
free, regardless of table size.
- **SIMD string scanning.** The JSON string decoder and encoder use an
AVX2 → SSE2 → SWAR cascade to skip over clean byte spans 32, 16, or 8
bytes at a time. The decoder scans for `"` and `\` (the only stop bytes
in clean strings); the encoder additionally detects control characters
(`c < 0x20`) via a bias trick that maps unsigned `< 0x20` to a signed
comparison, avoiding a branch-per-byte table lookup for the common
all-ASCII case. The same cascade is used by the CSV unquoted-field
scanner (`delimiter | LF | CR`) and the YAML double-quoted scalar scanner
(`"`, `\`, `LF`, `CR`), as well as single-character finders consolidated
in `glazer_common.hpp` (`find_byte`). On AVX2 hardware (Haswell+) this
processes up to 32 bytes per iteration instead of 1.
- **SWAR whitespace skipping.** `skip_ws` checks the next byte before
paying for any wider load, then — for runs of whitespace — scans 8 bytes
at a time using branch-free bit-twiddling ("SIMD within a register") to
find the first non-whitespace byte. Minified JSON (the overwhelmingly
common case) has little or no structural whitespace, so the single-byte
fast path dominates; the 8-byte path handles pretty-printed inputs.
- **Table-driven string escaping with bulk copies.** JSON string escaping
locates the next byte needing escaping in bulk (via the SIMD scanner
above), copies the clean prefix in one `memcpy`, then falls into a
per-byte switch only for the rare characters that actually need an escape
sequence.
- **Fast integer formatting.** Integers are written to JSON using a
lookup-table-based digit-pair algorithm (avoiding division for small
values) with a vendored `lltoa` fallback for larger numbers — faster
than routing every integer through `snprintf`.
## License
MIT License — see [LICENSE](LICENSE) for details.
Each chart compares glazer against other libraries for JSON/YAML/CSV
decode and encode on a representative small/medium/large file. Charts are
generated from the tables below via `scripts/gen_bench_charts.py`.
Benchmark tables:
- [Benchmarking JSON](#benchmarking-json)
- [Benchmarking YAML](#benchmarking-yaml)
- [Benchmarking CSV](#benchmarking-csv)