implement delta compression
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@ -6,6 +6,60 @@
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const std = @import("std");
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// delta compresses an incrementing sorted array of integers using delta
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// compression. Sorting is in-place.
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pub fn deltaCompress(comptime T: type, elems: []T) error{NotSorted}!void {
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if (elems.len == 0) {
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return;
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}
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var prev: T = elems[0];
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var i: usize = 1;
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while (i < elems.len) : (i += 1) {
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const cur = elems[i];
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if (cur <= prev) {
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return error.NotSorted;
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}
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elems[i] = cur - prev - 1;
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prev = cur;
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}
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}
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const testing = std.testing;
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test "delta compression positive tests" {
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const tests = [_]struct { input: []const u8, want: []const u8 }{
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.{ .input = &[_]u8{}, .want = &[_]u8{} },
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.{ .input = &[_]u8{0}, .want = &[_]u8{0} },
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.{ .input = &[_]u8{10}, .want = &[_]u8{10} },
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.{ .input = &[_]u8{ 0, 1, 2 }, .want = &[_]u8{ 0, 0, 0 } },
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.{ .input = &[_]u8{ 0, 254, 255 }, .want = &[_]u8{ 0, 253, 0 } },
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};
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for (tests) |t| {
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var arr = try std.ArrayList(u8).initCapacity(
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testing.allocator,
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t.input.len,
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);
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defer arr.deinit();
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try arr.appendSlice(t.input);
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try deltaCompress(u8, arr.items);
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try testing.expectEqualSlices(u8, arr.items, t.want);
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}
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}
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test "delta compression negative tests" {
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for ([_][]const u8{
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&[_]u8{ 0, 0 },
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&[_]u8{ 0, 1, 1 },
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&[_]u8{ 0, 1, 2, 1 },
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}) |t| {
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var arr = try std.ArrayList(u8).initCapacity(testing.allocator, t.len);
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defer arr.deinit();
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try arr.appendSlice(t);
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try testing.expectError(error.NotSorted, deltaCompress(u8, arr.items));
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}
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}
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// Represents a variable length integer that we read from a byte stream along
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// with how many bytes were read to decode it.
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pub const Varint = struct {
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@ -58,9 +112,8 @@ pub fn putUvarint(buf: []u8, x: u64) usize {
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return i + 1;
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}
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const testing = std.testing;
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const tests = [_]u64{
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test "uvarint" {
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const uvarint_tests = [_]u64{
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0,
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1,
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2,
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@ -77,9 +130,7 @@ const tests = [_]u64{
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257,
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1 << 63 - 1,
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};
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test "uvarint" {
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for (tests) |x| {
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for (uvarint_tests) |x| {
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var buf: [maxVarintLen64]u8 = undefined;
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const n = putUvarint(buf[0..], x);
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const got = try uvarint(buf[0..n]);
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