facet

import gleam/list
import gleam/option.{type Option, None, Some}
import gleam/string

pub type Error {
  Empty
  ParseError(msg: String)
}

/// The frontmatter content with format-specific variants.
pub type Frontmatter {
  TOML(content: String)
  JSON(content: String)
  YAML(content: String)
}

/// The Entire Document split between frontmatter and content.
pub type Document {
  Document(frontmatter: Option(Frontmatter), content: String)
}

fn grab_frontmatter(
  input: List(String),
  frontmatter: List(String),
  format_constructor: fn(String) -> Frontmatter,
  delimiter: String,
) -> Result(Document, Error) {
  case input {
    [] -> {
      Error(ParseError("expected ending delimitor"))
    }

    [line, ..rest] if line == delimiter -> {
      let final_content = case delimiter {
        "}" -> string.join(list.append(frontmatter, ["}"]), with: "\n")
        _ -> string.join(frontmatter, with: "\n")
      }
      Ok(Document(
        Some(format_constructor(final_content)),
        string.join(rest, with: "\n"),
      ))
    }

    [line, ..rest] ->
      grab_frontmatter(
        rest,
        list.append(frontmatter, [line]),
        format_constructor,
        delimiter,
      )
  }
}

/// Parse an input string and grab the frontmatter if it exists.
pub fn parse(in: String) -> Result(Document, Error) {
  let lines =
    in
    |> string.split("\n")

  case lines {
    [] -> {
      Error(Empty)
    }
    ["+++", ..rest] -> {
      grab_frontmatter(rest, [], TOML, "+++")
    }
    ["{", ..rest] -> {
      grab_frontmatter(rest, ["{"], JSON, "}")
    }
    ["---", ..rest] -> {
      grab_frontmatter(rest, [], YAML, "---")
    }
    rest -> {
      Ok(Document(None, string.join(rest, with: "\n")))
    }
  }
}