// The source code refers to the go standard library, which will be combined with AES in the future.
//
// Use of this source code is governed by an MIT license
// that can be found in the LICENSE file.
//
// OFB (Output Feedback) Mode.
// See NIST SP 800-38A, pp 13-15
module cipher

import crypto.internal.subtle

struct Ofb {
mut:
    b        Block
    next     []u8
    out      []u8
    out_used int
}

// new_ofb returns a Ofb that encrypts or decrypts using the block cipher b
// in output feedback mode. The initialization vector iv's length must be equal
// to b's block size.
pub fn new_ofb(b Block, iv []u8) Ofb {
    block_size := b.block_size
    if iv.len != block_size {
        panic('cipher.new_ofb: IV length must be equal block size')
    }
    mut x := Ofb{
        b:        b
        out:      []u8{len: b.block_size}
        next:     []u8{len: b.block_size}
        out_used: block_size
    }
    copy(mut x.next, iv)
    return x
}

// xor_key_stream xors each byte in the given slice with a byte from the key stream.
pub fn (mut x Ofb) xor_key_stream(mut dst []u8, src []u8) {
    unsafe {
        mut local_dst := *dst
        mut local_src := src
        if local_dst.len < local_src.len {
            panic('crypto.cipher.xor_key_stream: output smaller than input')
        }

        if subtle.inexact_overlap(local_dst[..local_src.len], local_src) {
            panic('crypto.cipher.xor_key_stream: invalid buffer overlap')
        }

        for local_src.len > 0 {
            if x.out_used == x.out.len {
                x.b.encrypt(mut x.out, x.next)
                x.out_used = 0
                copy(mut x.next, x.out)
            }
            n := xor_bytes(mut local_dst, local_src, x.out[x.out_used..])
            local_dst = local_dst[n..]
            local_src = local_src[n..]
            x.out_used += n
        }
    }
}