//! Functions for comparing files.

use std::fs::{metadata, File};
use std::io;
use std::io::{BufReader, Read};
use std::path::Path;

use difference;

/// A function that displays a diff and panics if two files to not match.
pub type Differ = Box<Fn(&Path, &Path)>;

/// Compare unicode text files. Print a colored diff and panic on failure.
pub fn text_diff(old: &Path, new: &Path) {
    difference::assert_diff!(&read_file(old), &read_file(new), "\n", 0);
}

/// Panic if binary files differ with some basic information about where they
/// differ.
pub fn binary_diff(old: &Path, new: &Path) {
    let old_len = file_len(old);
    let new_len = file_len(new);
    if old_len != new_len {
        panic!(
            "File sizes differ: Old file is {} bytes, new file is {} bytes",
            old_len, new_len
        );
    }

    let first_difference = file_byte_iter(old)
        .zip(file_byte_iter(new))
        .position(|(old_byte, new_byte)| old_byte != new_byte);

    if let Some(position) = first_difference {
        panic!("Files differ at byte {}", position + 1);
    }
}

fn open_file(path: &Path) -> File {
    check_io(File::open(path), "opening file", path)
}

fn file_byte_iter<'a>(path: &'a Path) -> impl Iterator<Item = u8> + 'a {
    BufReader::new(open_file(path))
        .bytes()
        .map(move |b| check_io(b, "reading file", path))
}

fn file_len(path: &Path) -> u64 {
    check_io(metadata(path), "getting file length", path).len()
}

fn read_file(path: &Path) -> String {
    let mut contents = String::new();
    check_io(
        open_file(path).read_to_string(&mut contents),
        "reading file",
        path,
    );
    contents
}

fn check_io<T>(x: Result<T, io::Error>, message: &str, path: &Path) -> T {
    x.unwrap_or_else(|_| panic!("Error {}: {:?}", message, path))
}