use std::fs::File;
use std::io::{BufRead, BufReader};

use super::error::LexerError;
use super::position::{Position, Span};
use super::token::{TokenStream, TokenType};



/// The size of data chunks to read from a file. This was arbitrarily chosen to
/// be 1mb.
const BUFFER_SIZE: usize = 1024 * 1024;



/// The `Lexer` is the core component responsible for performing
/// lexicographical analysis on a text file.
///
/// It reads input from a file character-by-character, generating a stream
/// of base tokens such as text, numbers, whitespace, symbols, and newlines.
/// These tokens are accumulated into a `TokenStream`, which is a flat,
/// cache-friendly data structure.
///
/// After tokenization, the lexer applies a user-provided `transform` function
/// to each token in the stream, allowing consumers of the library to convert
/// base tokens into richer, domain-specific token types (e.g. Markdown
/// elements, syntax trees, or custom DSL tokens).
///
/// # Example
///
/// ```rust
/// use rune::{Lexer, TokenStream, TokenType};
///
/// fn transform(tokens: &TokenStream<TokenType>) -> TokenStream<TokenType>
/// {
///    tokens.clone()
/// }
///
/// let tokens = Lexer::scan_text("Runes += 42", transform).unwrap();
///
/// // The tuple here is from the transform functions return type.
/// for token in &tokens
/// {
///    println!("{}", token);
/// }
/// ```
///
/// # Design Notes
///
/// - Uses a flat `TokenStream` to improve iteration performance and reduce heap
///   overhead.
/// - Consolidates contiguous characters into compound tokens (e.g. multi-digit
///   numbers).
/// - Easily extensible via the `transform` function to support higher-level
///   parsing tasks.
///
/// # Errors
///
/// Returns a `LexerError` if the file cannot be opened or read.
pub enum Lexer {}



impl Lexer
{
   /// Scans a file and produces a vector of transformed tokens.
   pub fn scan_file<P, F, T>(path: P, transform: F)
                             -> Result<TokenStream<T>, LexerError>
      where P: AsRef<std::path::Path>,
            F: FnOnce(&TokenStream<TokenType>) -> TokenStream<T>
   {
      let mut cursor = Position::default();
      let mut stream = TokenStream::new();

      let input_file = File::open(&path).map_err(|_error| {
                                           LexerError::new(
                "Unable to open file for Lexigraphical Analysis.",
                Span::default(),
                Some(path.as_ref().to_string_lossy().to_string()),
                None,
            )
                                        })?;

      let reader = BufReader::with_capacity(BUFFER_SIZE, input_file);

      for line in reader.lines()
      {
         match line
         {
            Ok(text) =>
            {
               Self::scan(&text, &mut stream, &mut cursor);
            }
            Err(_) =>
            {
               return Err(LexerError::new("Unable to read line during \
                                           Lexigraphical Analysis.",
                                          Span::default(),
                                          Some(path.as_ref()
                                                   .to_string_lossy()
                                                   .to_string()),
                                          None));
            }
         }

         stream.push("\n".to_string(),
                     TokenType::Newline,
                     Span::with_single(cursor));

         cursor.line += 1;
         cursor.column = 0;
      }

      Ok(transform(&stream))
   }

   /// Scans a full in-memory string and returns transformed tokens.
   pub fn scan_text<F, T>(text: &str, transform: F)
                          -> Result<TokenStream<T>, LexerError>
      where F: FnOnce(&TokenStream<TokenType>) -> TokenStream<T>
   {
      let mut cursor = Position::default();
      let mut stream = TokenStream::new();

      for line in text.lines()
      {
         Self::scan(line, &mut stream, &mut cursor);

         stream.push("\n".to_string(),
                     TokenType::Newline,
                     Span::with_single(cursor));

         cursor.line += 1;
         cursor.column = 0;
      }

      Ok(transform(&stream))
   }

   /// Internal: scans a single line of text into tokens.
   fn scan(line: &str, stream: &mut TokenStream<TokenType>, cursor: &mut Position)
   {
      for c in line.chars()
      {
         let variant = get_token_type(c);
         let last = stream.len().saturating_sub(1);

         if !stream.is_empty() &&
            variant == stream.variants[last] &&
            (variant == TokenType::Numeric || variant == TokenType::Text)
         {
            stream.lexemes[last].push(c);
            stream.locations[last].end = *cursor;
         }
         else
         {
            stream.push(c.to_string(), variant, Span::with_single(*cursor));
         }

         cursor.column += 1;
      }
   }
}



fn get_token_type(curr_char: char) -> TokenType
{
   match curr_char
   {
      '\n' => TokenType::Newline,
      c if c.is_whitespace() => TokenType::Whitespace,
      c if c.is_numeric() => TokenType::Numeric,
      c if c.is_alphabetic() => TokenType::Text,
      c if !c.is_whitespace() && !c.is_alphanumeric() => TokenType::Symbol,
      _ => TokenType::Unknown
   }
}