import gleam/dynamic.{type Dynamic}
import gleam/dynamic/decode.{type DecodeError, type Decoder}
import gleam/json
import gleam/option.{None}

import claude/types/content.{
  type Citation, type ContentBlock, CharLocationCitation,
  ContentBlockLocationCitation, PageLocationCitation, RedactedThinking,
  ServerToolUse, Text, Thinking, ToolUse, WebSearchResult,
  WebSearchResultLocationCitation,
}
import claude/types/message.{
  type Message, type Role, type StopReason, type Usage, Assistant, EndTurn,
  MaxTokens, Message, PauseTurn, Refusal, StopSequence, ToolUseStop, Usage, User,
}

/// Re-serialize a dynamic value (arbitrary JSON) back to a JSON string.
@external(erlang, "claude_json_ffi", "dynamic_to_json_string")
fn dynamic_to_json_string(term: Dynamic) -> String

/// Decode a full API response message from a JSON string.
pub fn message(
  json_string: String,
) -> Result(Message, List(DecodeError)) {
  json.parse(json_string, message_decoder())
  |> map_json_error
}

/// Map gleam_json's DecodeError to a list of dynamic DecodeErrors.
fn map_json_error(
  result: Result(a, json.DecodeError),
) -> Result(a, List(DecodeError)) {
  case result {
    Ok(v) -> Ok(v)
    Error(json.UnexpectedEndOfInput) ->
      Error([
        decode.DecodeError(
          expected: "valid JSON",
          found: "unexpected end of input",
          path: [],
        ),
      ])
    Error(json.UnexpectedByte(b)) ->
      Error([
        decode.DecodeError(
          expected: "valid JSON",
          found: "unexpected byte: " <> b,
          path: [],
        ),
      ])
    Error(json.UnexpectedSequence(s)) ->
      Error([
        decode.DecodeError(
          expected: "valid JSON",
          found: "unexpected sequence: " <> s,
          path: [],
        ),
      ])
    Error(json.UnableToDecode(errors)) -> Error(errors)
  }
}

/// Decoder for the top-level Message type.
pub fn message_decoder() -> Decoder(Message) {
  use id <- decode.field("id", decode.string)
  use role <- decode.field("role", role_decoder())
  use content <- decode.field("content", decode.list(content_block_decoder()))
  use model <- decode.field("model", decode.string)
  use stop_reason <- decode.field(
    "stop_reason",
    decode.optional(stop_reason_decoder()),
  )
  use stop_sequence <- decode.field(
    "stop_sequence",
    decode.optional(decode.string),
  )
  use usage <- decode.field("usage", usage_decoder())
  decode.success(Message(
    id: id,
    role: role,
    content: content,
    model: model,
    stop_reason: stop_reason,
    stop_sequence: stop_sequence,
    usage: usage,
  ))
}

/// Decoder for Role.
pub fn role_decoder() -> Decoder(Role) {
  use s <- decode.then(decode.string)
  case s {
    "user" -> decode.success(User)
    "assistant" -> decode.success(Assistant)
    _ -> decode.failure(User, "Role")
  }
}

/// Decoder for StopReason.
pub fn stop_reason_decoder() -> Decoder(StopReason) {
  use s <- decode.then(decode.string)
  case s {
    "end_turn" -> decode.success(EndTurn)
    "tool_use" -> decode.success(ToolUseStop)
    "max_tokens" -> decode.success(MaxTokens)
    "stop_sequence" -> decode.success(StopSequence)
    "pause_turn" -> decode.success(PauseTurn)
    "refusal" -> decode.success(Refusal)
    _ -> decode.failure(EndTurn, "StopReason")
  }
}

/// Decoder for Usage.
pub fn usage_decoder() -> Decoder(Usage) {
  use input_tokens <- decode.field("input_tokens", decode.int)
  use output_tokens <- decode.field("output_tokens", decode.int)
  use cache_creation <- decode.optional_field(
    "cache_creation_input_tokens",
    None,
    decode.optional(decode.int),
  )
  use cache_read <- decode.optional_field(
    "cache_read_input_tokens",
    None,
    decode.optional(decode.int),
  )
  decode.success(Usage(
    input_tokens: input_tokens,
    output_tokens: output_tokens,
    cache_creation_input_tokens: cache_creation,
    cache_read_input_tokens: cache_read,
  ))
}

/// Decoder for ContentBlock — discriminates on the "type" field.
fn content_block_decoder() -> Decoder(ContentBlock) {
  use block_type <- decode.field("type", decode.string)
  case block_type {
    "text" -> text_block_decoder()
    "thinking" -> thinking_block_decoder()
    "redacted_thinking" -> redacted_thinking_block_decoder()
    "tool_use" -> tool_use_block_decoder()
    "server_tool_use" -> server_tool_use_block_decoder()
    "web_search_result" -> web_search_result_block_decoder()
    _ -> decode.failure(Text(text: "", citations: []), "ContentBlock")
  }
}

/// Decoder for Text content block.
fn text_block_decoder() -> Decoder(ContentBlock) {
  use text <- decode.field("text", decode.string)
  use citations <- decode.optional_field(
    "citations",
    [],
    decode.list(citation_decoder()),
  )
  decode.success(Text(text: text, citations: citations))
}

/// Decoder for Citation — discriminates on the "type" field.
pub fn citation_decoder() -> Decoder(Citation) {
  use citation_type <- decode.field("type", decode.string)
  case citation_type {
    "char_location" -> {
      use cited_text <- decode.field("cited_text", decode.string)
      use document_index <- decode.field("document_index", decode.int)
      use start_char_index <- decode.field("start_char_index", decode.int)
      use end_char_index <- decode.field("end_char_index", decode.int)
      decode.success(CharLocationCitation(
        cited_text: cited_text,
        document_index: document_index,
        start_char_index: start_char_index,
        end_char_index: end_char_index,
      ))
    }
    "page_location" -> {
      use cited_text <- decode.field("cited_text", decode.string)
      use document_index <- decode.field("document_index", decode.int)
      use start_page <- decode.field("start_page", decode.int)
      use end_page <- decode.field("end_page", decode.int)
      decode.success(PageLocationCitation(
        cited_text: cited_text,
        document_index: document_index,
        start_page: start_page,
        end_page: end_page,
      ))
    }
    "content_block_location" -> {
      use cited_text <- decode.field("cited_text", decode.string)
      use document_index <- decode.field("document_index", decode.int)
      use start_block_index <- decode.field("start_block_index", decode.int)
      use end_block_index <- decode.field("end_block_index", decode.int)
      decode.success(ContentBlockLocationCitation(
        cited_text: cited_text,
        document_index: document_index,
        start_block_index: start_block_index,
        end_block_index: end_block_index,
      ))
    }
    "web_search_result_location" -> {
      use cited_text <- decode.field("cited_text", decode.string)
      use url <- decode.field("url", decode.string)
      use title <- decode.field("title", decode.string)
      decode.success(WebSearchResultLocationCitation(
        cited_text: cited_text,
        url: url,
        title: title,
      ))
    }
    _ ->
      decode.failure(
        CharLocationCitation(
          cited_text: "",
          document_index: 0,
          start_char_index: 0,
          end_char_index: 0,
        ),
        "Citation",
      )
  }
}

/// Decoder for Thinking content block.
fn thinking_block_decoder() -> Decoder(ContentBlock) {
  use thinking <- decode.field("thinking", decode.string)
  use signature <- decode.field("signature", decode.string)
  decode.success(Thinking(thinking: thinking, signature: signature))
}

/// Decoder for RedactedThinking content block.
fn redacted_thinking_block_decoder() -> Decoder(ContentBlock) {
  use data <- decode.field("data", decode.string)
  decode.success(RedactedThinking(data: data))
}

/// Decoder for ToolUse content block.
/// The "input" field is an arbitrary JSON object which we re-serialize to a string.
fn tool_use_block_decoder() -> Decoder(ContentBlock) {
  use id <- decode.field("id", decode.string)
  use name <- decode.field("name", decode.string)
  use input_dynamic <- decode.field("input", dynamic_decoder())
  // Re-serializing the already-parsed Dynamic value back to a JSON string for
  // storage in the ContentBlock. This string will be parsed again when dispatched
  // to a tool handler via tool.dispatch. This round-trip is a known trade-off of
  // the String-based ToolUse interface — it simplifies conversation history
  // encoding at the cost of an extra serialize-parse cycle.
  let input_string = dynamic_to_json_string(input_dynamic)
  decode.success(ToolUse(id: id, name: name, input: input_string))
}

/// Decoder for ServerToolUse content block.
fn server_tool_use_block_decoder() -> Decoder(ContentBlock) {
  use id <- decode.field("id", decode.string)
  use name <- decode.field("name", decode.string)
  use input_dynamic <- decode.field("input", dynamic_decoder())
  let input_string = dynamic_to_json_string(input_dynamic)
  decode.success(ServerToolUse(id: id, name: name, input: input_string))
}

/// Decoder for WebSearchResult content block.
fn web_search_result_block_decoder() -> Decoder(ContentBlock) {
  use id <- decode.field("id", decode.string)
  use title <- decode.field("title", decode.string)
  use url <- decode.field("url", decode.string)
  use encrypted_content <- decode.field("encrypted_content", decode.string)
  decode.success(WebSearchResult(
    id: id,
    title: title,
    url: url,
    encrypted_content: encrypted_content,
  ))
}

/// A decoder that returns the raw Dynamic value without transforming it.
/// This is used to capture the raw JSON value for ToolUse input.
fn dynamic_decoder() -> Decoder(Dynamic) {
  decode.new_primitive_decoder("Dynamic", fn(dyn) { Ok(dyn) })
}