const std = @import("std"); const mem = std.mem; pub const TransmissionError = error{ WrongParity, }; fn parity(data: u8) u8 { var n = data; var result: u8 = 0; while (n != 0) { n -= n & (~n + 1); result = 1 - result; } return result; } fn transmit(list: *std.array_list.Managed(u8), pending: u16) mem.Allocator.Error!void { const data = @as(u8, @intCast(pending & 0x7f)); try list.append((data << 1) | parity(data)); } pub fn transmitSequence(allocator: mem.Allocator, message: []const u8) mem.Allocator.Error![]u8 { var list = std.array_list.Managed(u8).init(allocator); defer list.deinit(); var count: u4 = 0; var pending: u16 = 0; for (0.., message) |i, byte| { pending = (pending << 8) | byte; count += 1; try transmit(&list, pending >> count); if (i + 1 == message.len) { pending <<= (7 - count); count = 7; } if (count == 7) { try transmit(&list, pending); count = 0; } std.debug.assert(count < 7); } std.debug.assert(count == 0); return try list.toOwnedSlice(); } pub fn decodeMessage(allocator: mem.Allocator, message: []const u8) (mem.Allocator.Error || TransmissionError)![]u8 { var list = std.array_list.Managed(u8).init(allocator); defer list.deinit(); var count: u4 = 0; var pending: u16 = 0; for (message) |byte| { if (parity(byte) != 0) { return TransmissionError.WrongParity; } pending = (pending << 7) | (byte >> 1); count += 7; if (count >= 8) { count -= 8; try list.append(@as(u8, @intCast((pending >> count) & 0xff))); } std.debug.assert(count < 8); } return try list.toOwnedSlice(); }