//! Unix-specific system calls and utilities.

use std::intrinsics;

/// File access modes.

export record OpenFlags(i64);

/// Open file for reading only.

export constant O_RDONLY: OpenFlags = OpenFlags(0);

/// Open file for writing only.

export constant O_WRONLY: OpenFlags = OpenFlags(1);

/// Open file for reading and writing.

export constant O_RDWR: OpenFlags = OpenFlags(2);

/// Create file if it doesn't exist.

export constant O_CREAT: OpenFlags = OpenFlags(64);

/// Truncate file to zero length.

export constant O_TRUNC: OpenFlags = OpenFlags(512);

/// Standard file descriptor for stdin.

export constant STDIN: i64 = 0;

/// Standard file descriptor for stdout.

export constant STDOUT: i64 = 1;

/// Standard file descriptor for stderr.

export constant STDERR: i64 = 2;

/// Special value representing current working directory for `openat()`.

constant AT_FDCWD: i64 = -100;

/// Open a file at the given path and return a file descriptor.

/// Return a negative value on error.

///

/// The caller must ensure `path.ptr` identifies at least `path.len + 1`

/// readable bytes, that `path.ptr[path.len]` is NUL, and that those bytes

/// remain readable until the system call returns.

export unsafe fn open(path: *[u8], flags: OpenFlags) -> i64 {

    return intrinsics::ecall(56, AT_FDCWD, path.ptr as i64, *flags, 0);

}

/// Open a file at the given path with mode and return a file descriptor.

///

/// The caller must ensure `path.ptr` identifies at least `path.len + 1`

/// readable bytes, that `path.ptr[path.len]` is NUL, and that those bytes

/// remain readable until the system call returns.

export unsafe fn openOpts(path: *[u8], flags: OpenFlags, mode: i64) -> i64 {

    return intrinsics::ecall(56, AT_FDCWD, path.ptr as i64, *flags, mode);

}

/// Reads from a file descriptor into the provided buffer.

/// Returns the number of bytes read, or a negative value on error.

export fn read(fd: i64, buf: *mut [u8]) -> i64 {

    let mut result: i64 = 0;

    unsafe { set result = intrinsics::ecall(63, fd, buf.ptr as i64, buf.len as i64, 0); }

    return result;

}

/// Reads from a file descriptor until EOF or buffer is full.

/// Returns the total number of bytes read, or a negative value on error.

export fn readToEnd(fd: i64, buf: *mut [u8]) -> i64 {

    let mut total: u32 = 0;

    while total < buf.len {

        let chunk = &mut buf[total..];

        let n = read(fd, chunk);

        if n < 0 {

            return n;

        }

        if n == 0 {

            break;

        }

        set total += n as u32;

    }

    return total as i64;

}

/// Writes to a file descriptor from the provided buffer.

/// Returns the number of bytes written, or a negative value on error.

export fn write(fd: i64, buf: *[u8]) -> i64 {

    let mut result: i64 = 0;

    unsafe { set result = intrinsics::ecall(64, fd, buf.ptr as i64, buf.len as i64, 0); }

    return result;

}

/// Writes the entire contents of a buffer to a file descriptor.

/// Returns `false` when the descriptor cannot accept the full buffer.

export fn writeAll(fd: i64, data: *[u8]) -> bool {

    let mut written: u32 = 0;

    while written < data.len {

        let n = write(fd, &data[written..]);

        if n <= 0 {

            return false;

        }

        set written += n as u32;

    }

    return true;

}

/// Closes a file descriptor.

/// Returns `0` on success, or a negative value on error.

export fn close(fd: i64) -> i64 {

    let mut result: i64 = 0;

    unsafe { set result = intrinsics::ecall(57, fd, 0, 0, 0); }

    return result;

}

/// Read the entire contents of a file at the given path into the buffer.

/// Return a slice containing the data read, or `nil` on error.

///

/// The caller must ensure `path.ptr` identifies at least `path.len + 1`

/// readable bytes, that `path.ptr[path.len]` is NUL, and that those bytes

/// remain readable until this function returns.

export unsafe fn readFile(path: *[u8], buf: *mut [u8]) -> ?*[u8] {

    let fd = open(path, O_RDONLY);

    if fd < 0 {

        return nil;

    }

    let n = readToEnd(fd, buf);

    if close(fd) < 0 {

        return nil;

    }

    if n < 0 {

        return nil;

    }

    return &buf[..n as u32];

}

/// Exit the current process with the given status code.

export fn exit(status: i64) {

    unsafe { intrinsics::ecall(93, status, 0, 0, 0); }

}

/// Write the entire contents of a buffer to a file at the given path.

/// Create the file if it does not exist and truncate it if it does.

/// Return `true` on success.

///

/// The caller must ensure `path.ptr` identifies at least `path.len + 1`

/// readable bytes, that `path.ptr[path.len]` is NUL, and that those bytes

/// remain readable until this function returns.

export unsafe fn writeFile(path: *[u8], data: *[u8]) -> bool {

    return writeFileParts(path, &[data]);

}

/// Write each buffer to a file at the given path.

/// Create the file if it does not exist and truncate it if it does.

/// Return `true` only when every part and the final close succeed.

///

/// The caller must ensure `path.ptr` identifies at least `path.len + 1`

/// readable bytes, that `path.ptr[path.len]` is NUL, and that those bytes

/// remain readable until this function returns.

export unsafe fn writeFileParts(path: *[u8], parts: *[*[u8]]) -> bool {

    let flags = OpenFlags(*O_WRONLY | *O_CREAT | *O_TRUNC);

    let fd = openOpts(path, flags, 420); // 0644 in octal.

    if fd < 0 {

        return false;

    }

    for part in parts {

        if not writeAll(fd, part) {

            close(fd);

            return false;

        }

    }

    return close(fd) == 0;

}