import gleam/dynamic/decode.{type Decoder}
import gleam/json
import gleam/list
import gleam/option.{type Option}
import gleam/string

import claude/json/decode as api_decode
import claude/types/content.{type ContentBlock, Text, ToolUse}
import claude/types/message.{type Message, type StopReason}

/// A delta in a streaming content block.
pub type Delta {
  TextDelta(String)
  InputJsonDelta(String)
  ThinkingDelta(String)
}

/// Usage information in a message_delta event.
pub type DeltaUsage {
  DeltaUsage(output_tokens: Int)
}

/// A streaming event from the Anthropic Messages API (SSE).
pub type StreamEvent {
  MessageStart(message: Message)
  ContentBlockStart(index: Int, content_block: ContentBlock)
  ContentBlockDelta(index: Int, delta: Delta)
  ContentBlockStop(index: Int)
  MessageDelta(stop_reason: Option(StopReason), usage: DeltaUsage)
  MessageStop
  Ping
  UnknownEvent(event_type: String, data: String)
}

/// Parse a full SSE text payload into a list of StreamEvents.
///
/// SSE events are separated by blank lines ("\n\n"). Each event block
/// contains an `event:` line and a `data:` line.
pub fn parse_sse(text: String) -> List(StreamEvent) {
  text
  |> string.split("\n\n")
  |> list.filter(fn(block) { !string.is_empty(string.trim(block)) })
  |> list.filter_map(parse_block)
}

/// Parse a single SSE block into a StreamEvent.
/// Returns Error(Nil) if the block cannot be parsed.
fn parse_block(block: String) -> Result(StreamEvent, Nil) {
  let lines = string.split(block, "\n")
  let event_type = find_field(lines, "event:")
  let data = find_field(lines, "data:")

  case event_type, data {
    Ok(evt), Ok(d) -> Ok(parse_event(evt, d))
    Ok(evt), Error(_) ->
      case evt {
        "ping" -> Ok(Ping)
        _ -> Ok(UnknownEvent(event_type: evt, data: ""))
      }
    _, _ -> Error(Nil)
  }
}

/// Extract the value after a field prefix (e.g. "event:" or "data:") from SSE lines.
fn find_field(lines: List(String), prefix: String) -> Result(String, Nil) {
  list.find_map(lines, fn(line) {
    let trimmed = string.trim(line)
    case string.starts_with(trimmed, prefix) {
      True -> {
        let value = string.drop_start(trimmed, string.length(prefix))
        Ok(string.trim(value))
      }
      False -> Error(Nil)
    }
  })
}

/// Parse a single event given its type and JSON data string.
pub fn parse_event(event_type: String, data: String) -> StreamEvent {
  case event_type {
    "message_start" -> parse_message_start(data)
    "content_block_start" -> parse_content_block_start(data)
    "content_block_delta" -> parse_content_block_delta(data)
    "content_block_stop" -> parse_content_block_stop(data)
    "message_delta" -> parse_message_delta(data)
    "message_stop" -> MessageStop
    "ping" -> Ping
    _ -> UnknownEvent(event_type: event_type, data: data)
  }
}

// --- Individual event parsers ---

fn parse_message_start(data: String) -> StreamEvent {
  let decoder = {
    use message <- decode.field("message", api_decode.message_decoder())
    decode.success(MessageStart(message: message))
  }
  case json.parse(data, decoder) {
    Ok(event) -> event
    Error(_) -> UnknownEvent(event_type: "message_start", data: data)
  }
}

fn parse_content_block_start(data: String) -> StreamEvent {
  let decoder = {
    use index <- decode.field("index", decode.int)
    use block <- decode.field("content_block", content_block_start_decoder())
    decode.success(ContentBlockStart(index: index, content_block: block))
  }
  case json.parse(data, decoder) {
    Ok(event) -> event
    Error(_) -> UnknownEvent(event_type: "content_block_start", data: data)
  }
}

fn parse_content_block_delta(data: String) -> StreamEvent {
  let decoder = {
    use index <- decode.field("index", decode.int)
    use delta <- decode.field("delta", delta_decoder())
    decode.success(ContentBlockDelta(index: index, delta: delta))
  }
  case json.parse(data, decoder) {
    Ok(event) -> event
    Error(_) -> UnknownEvent(event_type: "content_block_delta", data: data)
  }
}

fn parse_content_block_stop(data: String) -> StreamEvent {
  let decoder = {
    use index <- decode.field("index", decode.int)
    decode.success(ContentBlockStop(index: index))
  }
  case json.parse(data, decoder) {
    Ok(event) -> event
    Error(_) -> UnknownEvent(event_type: "content_block_stop", data: data)
  }
}

fn parse_message_delta(data: String) -> StreamEvent {
  let decoder = {
    use stop_reason <- decode.subfield(
      ["delta", "stop_reason"],
      decode.optional(api_decode.stop_reason_decoder()),
    )
    use output_tokens <- decode.subfield(
      ["usage", "output_tokens"],
      decode.int,
    )
    decode.success(MessageDelta(
      stop_reason: stop_reason,
      usage: DeltaUsage(output_tokens: output_tokens),
    ))
  }
  case json.parse(data, decoder) {
    Ok(event) -> event
    Error(_) -> UnknownEvent(event_type: "message_delta", data: data)
  }
}

// --- Decoders ---

/// Decoder for a content block in content_block_start events.
/// Handles "text" and "tool_use" types.
fn content_block_start_decoder() -> Decoder(ContentBlock) {
  use block_type <- decode.field("type", decode.string)
  case block_type {
    "text" -> {
      use text <- decode.field("text", decode.string)
      decode.success(Text(text: text, citations: []))
    }
    "tool_use" -> {
      use id <- decode.field("id", decode.string)
      use name <- decode.field("name", decode.string)
      decode.success(ToolUse(id: id, name: name, input: ""))
    }
    _ ->
      decode.success(Text(text: "", citations: []))
  }
}

/// Decoder for the delta object in content_block_delta events.
fn delta_decoder() -> Decoder(Delta) {
  use delta_type <- decode.field("type", decode.string)
  case delta_type {
    "text_delta" -> {
      use text <- decode.field("text", decode.string)
      decode.success(TextDelta(text))
    }
    "input_json_delta" -> {
      use partial_json <- decode.field("partial_json", decode.string)
      decode.success(InputJsonDelta(partial_json))
    }
    "thinking_delta" -> {
      use thinking <- decode.field("thinking", decode.string)
      decode.success(ThinkingDelta(thinking))
    }
    _ -> decode.success(TextDelta(""))
  }
}