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Overhauled leb128:

Browse source 
handles integers < 8 bits
    incorrect overflow bugs fixed
    simplified *mem implementations
    added wrte* functions
    added thurough write/read testing
This commit is contained in:
tgschultz 2020-06-11 18:42:56 +00:00
parent 928d3ee9ea
commit 00ec81b0dc
1 changed files with 34 additions and 45 deletions
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79
lib/std/debug/leb128.zig
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@ -1,8 +1,11 @@
const std = @import("std");
const testing = std.testing;
///Read a single unsigned LEB128 value from the given reader as type T,
/// or error.Overflow if the value cannot fit.
//@TODO: you can take *slice and alter slice.ptr
// make sign bits check more efficient
// add wrapper readLEB128 and write LEB128 that infer from type?
// or use assertions?
pub fn readULEB128(comptime T: type, reader: var) !T {
const U = if (T.bit_count < 8) u8 else T;
const ShiftT = std.math.Log2Int(U);
@ -25,14 +28,11 @@ pub fn readULEB128(comptime T: type, reader: var) !T {
}
//only applies in the case that we extended to u8
if (U != T) {
if (value > std.math.maxInt(T)) return error.Overflow;
}
if (value > std.math.maxInt(T)) return error.Overflow;
return @truncate(T, value);
}
///Write a single unsigned integer as unsigned LEB128 to the given writer.
pub fn writeULEB128(writer: var, uint_value: var) !void {
const T = @TypeOf(uint_value);
const U = if (T.bit_count < 8) u8 else T;
@ -50,27 +50,22 @@ pub fn writeULEB128(writer: var, uint_value: var) !void {
}
}
///Read a single unsinged integer from the given memory as type T.
/// The provided slice reference will be updated to point to the byte after the last byte read.
pub fn readULEB128Mem(comptime T: type, ptr: *[]const u8) !T {
var buf = std.io.fixedBufferStream(ptr.*);
pub fn readULEB128Mem(comptime T: type, ptr: *[*]const u8) !T {
const max_group = (T.bit_count + 6) / 7;
var buf = std.io.fixedBufferStream(ptr.*[0 .. max_group + 1]);
const value = try readULEB128(T, buf.reader());
ptr.*.ptr += buf.pos;
ptr.* += @intCast(usize, try buf.getPos());
return value;
}
///Write a single unsigned LEB128 integer to the given memory as unsigned LEB128,
/// returning the number of bytes written.
pub fn writeULEB128Mem(ptr: []u8, uint_value: var) !usize {
const T = @TypeOf(uint_value);
const max_group = (T.bit_count + 6) / 7;
var buf = std.io.fixedBufferStream(ptr);
try writeULEB128(buf.writer(), uint_value);
return buf.pos;
return try buf.getPos();
}
///Read a single signed LEB128 value from the given reader as type T,
/// or error.Overflow if the value cannot fit.
pub fn readILEB128(comptime T: type, reader: var) !T {
const S = if (T.bit_count < 8) i8 else T;
const U = std.meta.Int(false, S.bit_count);
@ -85,24 +80,23 @@ pub fn readILEB128(comptime T: type, reader: var) !T {
const byte = try reader.readByte();
var temp = @as(U, byte & 0x7f);
const shift = group * 7;
if (@shlWithOverflow(U, temp, shift, &temp)) {
if (@shlWithOverflow(U, temp, group * 7, &temp)) {
//Overflow is ok so long as the sign bit is set and this is the last byte
if (byte & 0x80 != 0) return error.Overflow;
if (@bitCast(S, temp) >= 0) return error.Overflow;
//and all the overflowed bits are 1
const remaining_shift = @intCast(u3, U.bit_count - @as(u16, shift));
const remaining_bits = @bitCast(i8, byte | 0x80) >> remaining_shift;
if (remaining_bits != -1) return error.Overflow;
const check_bits_shift = @intCast(u3, U.bit_count - @as(u16, group * 7));
const check_bits_remaining = 7 - check_bits_shift;
const check_bits = byte >> check_bits_shift;
const num_consecutive_ones = @ctz(u8, ~check_bits);
if (num_consecutive_ones < check_bits_remaining) return error.Overflow;
}
value |= temp;
if (byte & 0x80 == 0) {
const needs_sign_ext = group + 1 < max_group;
if (byte & 0x40 != 0 and needs_sign_ext) {
const ones = @as(S, -1);
value |= @bitCast(U, ones) << (shift + 7);
if (byte & 0x40 != 0 and group + 1 < max_group) {
value |= @bitCast(U, @as(S, -1)) << ((group + 1) * 7);
}
break;
}
@ -110,16 +104,12 @@ pub fn readILEB128(comptime T: type, reader: var) !T {
return error.Overflow;
}
const result = @bitCast(S, value);
//Only applies if we extended to i8
if (S != T) {
if (result > std.math.maxInt(T) or result < std.math.minInt(T)) return error.Overflow;
}
if (@bitCast(S, value) > std.math.maxInt(T) or @bitCast(S, value) < std.math.minInt(T)) return error.Overflow;
return @truncate(T, result);
return @truncate(T, @bitCast(S, value));
}
///Write a single signed integer as signed LEB128 to the given writer.
pub fn writeILEB128(writer: var, int_value: var) !void {
const T = @TypeOf(int_value);
const S = if (T.bit_count < 8) i8 else T;
@ -141,22 +131,19 @@ pub fn writeILEB128(writer: var, int_value: var) !void {
}
}
///Read a single singed LEB128 integer from the given memory as type T.
/// The provided slice reference will be updated to point to the byte after the last byte read.
pub fn readILEB128Mem(comptime T: type, ptr: *[]const u8) !T {
var buf = std.io.fixedBufferStream(ptr.*);
pub fn readILEB128Mem(comptime T: type, ptr: *[*]const u8) !T {
const max_group = (T.bit_count + 6) / 7;
var buf = std.io.fixedBufferStream(ptr.*[0 .. max_group + 1]);
const value = try readILEB128(T, buf.reader());
ptr.*.ptr += buf.pos;
ptr.* += @intCast(usize, try buf.getPos());
return value;
}
///Write a single signed LEB128 integer to the given memory as unsigned LEB128,
/// returning the number of bytes written.
pub fn writeILEB128Mem(ptr: []u8, int_value: var) !usize {
const T = @TypeOf(int_value);
var buf = std.io.fixedBufferStream(ptr);
try writeILEB128(buf.writer(), int_value);
return buf.pos;
return try buf.getPos();
}
//tests
@ -173,7 +160,7 @@ fn test_read_stream_uleb128(comptime T: type, encoded: []const u8) !T {
fn test_read_ileb128(comptime T: type, encoded: []const u8) !T {
var reader = std.io.fixedBufferStream(encoded);
const v1 = try readILEB128(T, reader.reader());
var in_ptr = encoded;
var in_ptr = encoded.ptr;
const v2 = try readILEB128Mem(T, &in_ptr);
testing.expectEqual(v1, v2);
return v1;
@ -182,7 +169,7 @@ fn test_read_ileb128(comptime T: type, encoded: []const u8) !T {
fn test_read_uleb128(comptime T: type, encoded: []const u8) !T {
var reader = std.io.fixedBufferStream(encoded);
const v1 = try readULEB128(T, reader.reader());
var in_ptr = encoded;
var in_ptr = encoded.ptr;
const v2 = try readULEB128Mem(T, &in_ptr);
testing.expectEqual(v1, v2);
return v1;
@ -190,7 +177,7 @@ fn test_read_uleb128(comptime T: type, encoded: []const u8) !T {
fn test_read_ileb128_seq(comptime T: type, comptime N: usize, encoded: []const u8) !void {
var reader = std.io.fixedBufferStream(encoded);
var in_ptr = encoded;
var in_ptr = encoded.ptr;
var i: usize = 0;
while (i < N) : (i += 1) {
const v1 = try readILEB128(T, reader.reader());
@ -201,7 +188,7 @@ fn test_read_ileb128_seq(comptime T: type, comptime N: usize, encoded: []const u
fn test_read_uleb128_seq(comptime T: type, comptime N: usize, encoded: []const u8) !void {
var reader = std.io.fixedBufferStream(encoded);
var in_ptr = encoded;
var in_ptr = encoded.ptr;
var i: usize = 0;
while (i < N) : (i += 1) {
const v1 = try readULEB128(T, reader.reader());
@ -298,6 +285,8 @@ test "deserialize unsigned LEB128" {
fn test_write_leb128(value: var) !void {
const T = @TypeOf(value);
if (T.bit_count == 0) std.debug.warn("{}\n", .{@typeName(T)});
const writeStream = if (T.is_signed) writeILEB128 else writeULEB128;
const writeMem = if (T.is_signed) writeILEB128Mem else writeULEB128Mem;
const readStream = if (T.is_signed) readILEB128 else readULEB128;
@ -335,13 +324,13 @@ fn test_write_leb128(value: var) !void {
//mem read
var buf_ref: []u8 = buf[0..];
const mr = try readMem(T, &buf_ref);
const mr = try readMem(T, &buf_ref.ptr);
testing.expect(@ptrToInt(buf_ref.ptr) - @ptrToInt(&buf) == w2_pos);
testing.expect(mr == value);
//bigger type mem read
buf_ref = buf[0..];
const bmr = try readMem(T, &buf_ref);
const bmr = try readMem(T, &buf_ref.ptr);
testing.expect(@ptrToInt(buf_ref.ptr) - @ptrToInt(&buf) == w2_pos);
testing.expect(bmr == value);
}