defmodule ExCellerate do
  @moduledoc """
  ExCellerate is a high-performance expression evaluation engine for Elixir.

  It parses text expressions into an intermediate representation (IR) and then
  compiles them into native Elixir AST for near-native performance.

  ## Operators

  - **Arithmetic**: `+`, `-`, `*`, `/`, `^` (power), `%` (modulo), `n!` (factorial)
  - **Comparison**: `==`, `!=`, `<`, `<=`, `>`, `>=`
  - **Logical**: `&&`, `||`, `not`
  - **Bitwise**: `&`, `|`, `|^` (xor), `<<`, `>>`, `~` (bnot)
  - **Ternary**: `condition ? true_val : false_val`
  - **Data access**: `user.profile.name`, `list[0]`, `list[*].field` (spread)

  ## Built-in Functions

  ### Math

  | Function | Description |
  |----------|-------------|
  | `abs(n)` | Absolute value |
  | `round(n)` or `round(n, digits)` | Rounds to nearest integer, or to `digits` decimal places |
  | `floor(n)` | Largest integer ≤ `n` |
  | `ceil(n)` | Smallest integer ≥ `n` |
  | `trunc(n)` | Truncates toward zero (unlike `floor` for negatives) |
  | `max(a, b, ...)` or `max(list)` | Maximum of arguments or a list |
  | `min(a, b, ...)` or `min(list)` | Minimum of arguments or a list |
  | `sign(n)` | Returns -1, 0, or 1 |
  | `sqrt(n)` | Square root |
  | `exp(n)` | e raised to the power `n` |
  | `ln(n)` | Natural logarithm (base e) |
  | `log(n, base)` | Logarithm with specified base |
  | `log10(n)` | Base-10 logarithm |
  | `sum(a, b, ...)` or `sum(list)` | Sums arguments or a list |
  | `avg(a, b, ...)` or `avg(list)` | Arithmetic mean of arguments or a list |

  ### String

  | Function | Description |
  |----------|-------------|
  | `len(s)` or `len(list)` | String length or list length |
  | `left(s)` or `left(s, n)` | First character, or first `n` characters |
  | `right(s)` or `right(s, n)` | Last character, or last `n` characters |
  | `substring(s, start)` | Substring from `start` to end |
  | `substring(s, start, len)` | Substring of `len` characters |
  | `upper(s)` | Converts to uppercase |
  | `lower(s)` | Converts to lowercase |
  | `trim(s)` | Removes leading/trailing whitespace |
  | `concat(a, b, ...)` | Concatenates values into a string |
  | `textjoin(delim, a, b, ...)` | Joins values with a delimiter |
  | `replace(s, old, new)` | Replaces all occurrences of `old` with `new` |
  | `find(search, text)` or `find(search, text, start)` | 0-based position of `search` in `text`, optionally from `start` |
  | `contains(s, term)` | Returns `true` if `term` exists within `s` |
  | `underscore(s)` | Downcases, replaces spaces/slashes with underscores, strips special chars |
  | `slug(s)` | Downcases, replaces spaces/slashes with hyphens, strips special chars |

  ### Utility

  | Function | Description |
  |----------|-------------|
  | `if(cond, t)` or `if(cond, t, f)` | Returns `t` if truthy; `f` (or `nil`) otherwise |
  | `ifs(c1, v1, c2, v2, ...)` | Returns value for first truthy condition; `nil` if none match |
  | `ifnull(val, default)` | Returns `default` if `val` is nil |
  | `isnull(val)` | Returns `true` if `val` is nil, `false` otherwise |
  | `isblank(val)` | Returns `true` if `val` is nil or a whitespace-only string |
  | `coalesce(a, b, ...)` | Returns the first non-nil value |
  | `switch(expr, c1, v1, ..., default)` | Multi-way value matching |
  | `and(a, b, ...)` | Returns `true` if all arguments are truthy |
  | `or(a, b, ...)` | Returns `true` if any argument is truthy |
  | `lookup(coll, key)` | Looks up `key` in a map or index in a list |
  | `lookup(coll, key, default)` | Same, with a default for missing keys |
  | `match(value, list)` | Returns the 0-based position of `value` in `list` (exact match) |
  | `match(value, list, type)` | Approximate match: `1` for ascending (<=), `-1` for descending (>=), `0` for exact |
  | `index(list, row)` | Returns the element at 0-based position `row` |
  | `index(array, row, col)` | Returns the element at `row` and `col` in a 2D array |
  | `filter(list, predicates)` | Returns items where predicate is `true` |
  | `table(key1, list1, ...)` | Builds a list of maps from key/list pairs |
  | `take(list, rows)` | Takes the first/last `rows` elements (negative counts from end) |
  | `take(list, rows, cols)` | Takes rows and columns from a 2D array; pass `null` to skip a dimension |
  | `slice(list, start)` | Returns elements from `start` index to end (negative counts from end) |
  | `slice(list, start, len)` | Returns `len` elements starting at `start` |

  ### Special Forms

  | Form | Description |
  |------|-------------|
  | `let(name, value, expr)` | Lexically binds `name` within `expr` only |

  Custom functions can be added via the `ExCellerate.Registry` system.

  ## Multi-line Expressions

  Expressions can be formatted across multiple lines for readability.
  Newlines are treated as whitespace by the parser:

      expr = \"\"\"
      ifs(
        score > 90, 'A',
        score > 80, 'B',
        score > 70, 'C',
        true, 'F'
      )
      \"\"\"

      ExCellerate.eval!(expr, %{"score" => 85})
      # => "B"

  This works with `validate/2` and `compile/2` as well.

  ## Nil Propagation

  Dot and bracket access use **nil propagation**: if any key in a path is
  missing or the target is `nil`, the entire expression returns `nil` instead
  of raising an error. This mirrors how spreadsheets treat empty cells and
  avoids the need for defensive checks at every level of a nested path.

      ExCellerate.eval!("user.profile.name", %{"user" => %{}})
      # => nil  (profile is missing, so .name is never attempted)

      ExCellerate.eval!("user.name", %{"user" => nil})
      # => nil  (user is nil, short-circuits)

      ExCellerate.eval!("list[99]", %{"list" => [1, 2, 3]})
      # => nil  (index out of bounds)

  Use `ifnull/2`, `coalesce/2+`, or a ternary to provide defaults:

      ExCellerate.eval!("ifnull(user.name, 'anonymous')", %{"user" => %{}})
      # => "anonymous"

  **Root variable lookup still raises.** Referencing a variable that doesn't
  exist in the scope at all (e.g., `unknown_var`) is treated as a likely typo
  and returns `{:error, %ExCellerate.Error{}}`:

      ExCellerate.eval("totally_unknown", %{})
      # => {:error, %ExCellerate.Error{message: "variable not found: totally_unknown"}}

  ## Examples

      iex> ExCellerate.eval!("1 + 2 * 3")
      7

      iex> ExCellerate.eval("a + b", %{"a" => 10, "b" => 20})
      {:ok, 30}

      iex> ExCellerate.eval!("user.name", %{"user" => %{"name" => "Alice"}})
      "Alice"

  ## Column Spread (`[*]`)

  The `[*]` operator extracts a field from every element of a list,
  returning a new list of values. This enables column-oriented operations
  on lists of maps, structs, or nested lists:

      # Given a list of maps in scope
      scope = %{"orders" => [
        %{"product" => "Widget", "price" => 10, "qty" => 2},
        %{"product" => "Gadget", "price" => 25, "qty" => 1}
      ]}

      ExCellerate.eval!("orders[*].product", scope)
      # => ["Widget", "Gadget"]

      ExCellerate.eval!("sum(orders[*].price)", scope)
      # => 35

  Spread results work with any function that accepts a list: `sum`, `avg`,
  `max`, `min`, `len`, and `textjoin`. Subsequent access chains apply to
  each element:

      ExCellerate.eval!("orders[*].price", scope)       # => [10, 25]
      ExCellerate.eval!("users[*].profile.name", scope)  # deep access

  Bracket indexing after a spread selects a specific position from each
  element's sub-list:

      # scores[1] from each row
      ExCellerate.eval!("rows[*].scores[1]", scope)

   Nested `[*]` operators flatten across levels:

       # departments[*].employees[*].name => all employee names, flattened

  ### Computed Spread (`.(expr)`)

  To evaluate an expression *per element* of a spread, use the `.(expr)`
  syntax. Inside the parentheses, bare variable names resolve against each
  element rather than the outer scope:

      ExCellerate.eval!("orders[*].(qty * price)", scope)
      # => [20, 25]   (per-row product)

      ExCellerate.eval!("sum(orders[*].(qty * price))", scope)
      # => 45          (total of per-row products)

  Computed spreads also compose with nested `[*]`:

      # departments[*].employees[*].(salary * 12)  => annualised salaries, flattened

  ## Let, Filter, and Table

  `let/3` introduces a lexical binding that is visible only inside the body
  expression; it does not mutate the outer scope:

      # Given scope = %{"orders" => [%{"price" => 10, "qty" => 2}, ...]}
      let(total, sum(orders[*].price), total + 5)

  `filter/2` selects items from a list using a boolean list produced by a
  computed spread. The predicate list must be the same length as the input list:

      filter(orders, orders[*].(qty > 1))

  `table` builds a list of maps from alternating key/list pairs. Use spread
  or computed spread to produce the list columns:

      table('product', orders[*].product, 'total', orders[*].(qty * price))
      # => [%{"product" => "Widget", "total" => 20}, ...]

  These compose naturally — filter first, then build a summary table:

      let(big, filter(orders, orders[*].(qty > 1)),
        table('product', big[*].product, 'total', big[*].(qty * price)))

  ## Take and Slice

  `take/2` and `take/3` extract rows, columns, or both from a list or 2D
  array. Positive counts take from the beginning, negative from the end.
  Pass `null` to skip a dimension:

      take(data, 3)          # first 3 rows
      take(data, -3)         # last 3 rows
      take(data, null, 2)    # first 2 columns (all rows)
      take(data, 2, 2)       # first 2 rows and 2 columns

  `slice/2` and `slice/3` extract a contiguous section of a list by
  start index and optional length. The start index is zero-based, and
  negative indices count from the end:

      slice(items, 1)        # from index 1 to end
      slice(items, 1, 3)     # 3 elements starting at index 1
      slice(items, -2)       # last 2 elements
  """

  alias ExCellerate.Compiler
  alias ExCellerate.Parser

  @type scope :: %{optional(String.t()) => any()}
  @type registry :: module() | nil

  @doc """
  Evaluates a text expression against an optional scope and registry.

  Returns `{:ok, result}` on success or `{:error, %ExCellerate.Error{}}` on failure.

  ## Parameters

  - `expression` — a string containing the expression.
  - `scope` — a map of variables available to the expression. Supports string
    keys, atom keys, and structs. Defaults to `%{}`.
  - `registry` — an optional module created with `use ExCellerate.Registry`.

  ## Examples

      iex> ExCellerate.eval("1 + 2 * 3")
      {:ok, 7}

      iex> ExCellerate.eval("a + b", %{"a" => 10, "b" => 20})
      {:ok, 30}

      iex> ExCellerate.eval("user.name", %{"user" => %{"name" => "Alice"}})
      {:ok, "Alice"}

      iex> {:error, _} = ExCellerate.eval("1 + * 2")

  """
  @spec eval(String.t(), scope(), registry()) :: {:ok, any()} | {:error, Exception.t()}
  def eval(expression, scope \\ %{}, registry \\ nil) do
    case compile(expression, registry) do
      {:ok, fun} ->
        try do
          {:ok, fun.(scope)}
        rescue
          e -> {:error, e}
        end

      {:error, reason} ->
        {:error, reason}
    end
  end

  @doc """
  Similar to `eval/3`, but returns the result directly or raises on error.

  ## Examples

      iex> ExCellerate.eval!("1 + 2 * 3")
      7

      iex> ExCellerate.eval!("a > 10 ? 'high' : 'low'", %{"a" => 15})
      "high"

      iex> ExCellerate.eval!("concat('Hello', ' ', name)", %{"name" => "Alice"})
      "Hello Alice"

      iex> ExCellerate.eval!("user.profile.id", %{"user" => %{"profile" => %{"id" => 1}}})
      1

  """
  @spec eval!(String.t(), scope(), registry()) :: any()
  def eval!(expression, scope \\ %{}, registry \\ nil) do
    case eval(expression, scope, registry) do
      {:ok, result} -> result
      {:error, error} -> raise error
    end
  end

  @doc """
  Compiles an expression into a reusable function.

  Returns `{:ok, fun}` where `fun` is a 1-arity function that accepts a
  scope map and returns the result. The compiled function is cached, so
  calling `compile/2` repeatedly with the same expression is cheap.

  This is useful when you need to evaluate the same expression many times
  with different scopes — the parsing and compilation happen only once.

  ## Examples

      iex> {:ok, fun} = ExCellerate.compile("a + b")
      iex> fun.(%{"a" => 1, "b" => 2})
      3

  """
  @spec compile(String.t(), registry()) ::
          {:ok, (scope() -> any())} | {:error, ExCellerate.Error.t()}
  def compile(expression, registry \\ nil) do
    case ExCellerate.Cache.get(registry, expression) do
      {:ok, fun} ->
        {:ok, fun}

      :error ->
        case compile_to_function(expression, registry) do
          {:ok, fun} ->
            ExCellerate.Cache.put(registry, expression, fun)
            {:ok, fun}

          {:error, _} = error ->
            error
        end
    end
  end

  @doc """
  Similar to `compile/2`, but returns the function directly or raises on error.

  ## Examples

      iex> fun = ExCellerate.compile!("x * 2")
      iex> fun.(%{"x" => 5})
      10

  """
  @spec compile!(String.t(), registry()) :: (scope() -> any())
  def compile!(expression, registry \\ nil) do
    case compile(expression, registry) do
      {:ok, fun} -> fun
      {:error, error} -> raise error
    end
  end

  @doc """
  Validates that an expression is syntactically correct and all referenced
  functions exist in the registry or defaults.

  Returns `:ok` if the expression is valid, or `{:error, %ExCellerate.Error{}}`
  if it contains syntax errors or references unknown functions.

  **Note:** Validation checks syntax, function existence, and arity, but does
  not perform type checking. Since scope values are not known until runtime,
  type errors (e.g., passing a number to `upper/1`) will only be caught at
  evaluation time.

  ## Examples

      iex> ExCellerate.validate("1 + 2")
      :ok

      iex> ExCellerate.validate("abs(-1)")
      :ok

      iex> {:error, _} = ExCellerate.validate("1 +")

      iex> {:error, _} = ExCellerate.validate("unknown_func(1)")

  """
  @spec validate(String.t(), registry()) :: :ok | {:error, ExCellerate.Error.t()}
  def validate(expression, registry \\ nil) do
    case compile(expression, registry) do
      {:ok, _fun} -> :ok
      {:error, reason} -> {:error, reason}
    end
  end

  # Parses and compiles an expression string into a reusable function.
  # The function accepts a scope map and returns the evaluation result.
  # The scope parameter must use var!(scope) in the Compiler context
  # to match the references generated by Compiler.compile/2.
  defp compile_to_function(expression, registry) do
    case Parser.parse(expression) do
      {:ok, ast} ->
        try do
          elixir_ast = Compiler.compile(ast, registry)
          scope_var = Compiler.scope_var()

          fun_ast =
            {:fn, [], [{:->, [], [[scope_var], elixir_ast]}]}

          {fun, _} = Code.eval_quoted(fun_ast, [], __ENV__)
          {:ok, fun}
        rescue
          e -> {:error, e}
        end

      {:error, reason} ->
        {:error, reason}
    end
  end
end