//! 从 QR 矩阵数据区域蛇形读取码字比特
//!
//! 读取顺序严格对应 `placement.rs::place_data` 的写入顺序：
//! 从右下角开始，两列一组上下交替扫描，跳过保留模块。

use crate::matrix::grid::Matrix;

/// 从矩阵非保留区域蛇形读取码字比特
fn extract_bits(matrix: &Matrix, total_codewords: usize) -> Vec<bool> {
    let size = matrix.size as usize;
    let target_bits = total_codewords * 8;
    let mut bits: Vec<bool> = Vec::with_capacity(target_bits);
    let mut col = (size - 1) as i16;
    let mut going_up = true;

    while col >= 0 && bits.len() < target_bits {
        let actual_col = col as usize;
        if going_up {
            for row in (0..size).rev() {
                read_module(matrix, &mut bits, actual_col, row);
                if actual_col > 0 {
                    read_module(matrix, &mut bits, actual_col - 1, row);
                }
            }
        } else {
            for row in 0..size {
                read_module(matrix, &mut bits, actual_col, row);
                if actual_col > 0 {
                    read_module(matrix, &mut bits, actual_col - 1, row);
                }
            }
        }
        col -= 2;
        going_up = !going_up;
        // 垂直时序图案列（col 6）由 read_module 自动跳过保留区，
        // 无需显式 skip，否则会导致列配对错位、行序反转
    }

    bits.truncate(target_bits);
    bits
}

/// 读取单个非保留模块，追加 bool 到 bits
fn read_module(matrix: &Matrix, bits: &mut Vec<bool>, x: usize, y: usize) {
    let xu = x as u8;
    let yu = y as u8;
    if xu < matrix.size && yu < matrix.size && !matrix.is_reserved(xu, yu) {
        bits.push(matrix.get(xu, yu));
    }
}

/// 将布尔比特打包为 u8 码字（MSB 优先）
fn bits_to_bytes(bits: &[bool]) -> Vec<u8> {
    bits.chunks(8)
        .map(|chunk| {
            let mut byte = 0u8;
            for &b in chunk {
                byte = (byte << 1) | (b as u8);
            }
            // 不足 8 位的 chunk 左对齐
            byte <<= 8 - chunk.len();
            byte
        })
        .collect()
}

/// 从矩阵提取数据码字
///
/// *`total_codewords` 为数据码字总数（来自 EcInfo.total_codewords）*
pub(crate) fn extract_codewords(matrix: &Matrix, total_codewords: usize) -> Vec<u8> {
    let bits = extract_bits(matrix, total_codewords);
    bits_to_bytes(&bits)
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::qr::{QrCode, QrConfig};
    use crate::version::get_data_capacity;

    #[test]
    fn test_extract_roundtrip_simple() {
        let qr = QrCode::encode("HELLO WORLD", QrConfig::default()).unwrap();
        let cap = get_data_capacity(qr.version, qr.level) as usize;
        let codewords = extract_codewords(qr.matrix(), cap);
        assert!(!codewords.is_empty());
        // 前 11 字节编码 "HELLO WORLD"（byte 模式 4b+8b+88b=100bits≈13字节）
        assert!(codewords.len() >= 5); // 至少有数据
    }
}