Custom Level

examples/custom_level.zig - i32 levels (not enum) via zstd.compressWithLevel.

Client Code

const std = @import("std");
const zstd = @import("zstd");

pub fn main() !void {
    var gpa = std.heap.DebugAllocator(.{}){};
    defer _ = gpa.deinit();
    const allocator = gpa.allocator();

    var data: std.ArrayList(u8) = .empty;
    defer data.deinit(allocator);
    for (0..20) |_| try data.appendSlice(allocator, "Text with moderate repetitiveness for level testing. ");

    for ([_]i32{ 1, 3, 6, 9, 15, 19 }) |level| {
        const c = try zstd.compressWithLevel(allocator, data.items, level);
        defer allocator.free(c);
        std.debug.print("Level {d}: {d} -> {d} bytes ({d:.1}%)\n", .{ level, data.items.len, c.len, @as(f64, @floatFromInt(c.len)) / @as(f64, @floatFromInt(data.items.len)) * 100 });
        const d = try zstd.decompress(allocator, c);
        defer allocator.free(d);
        std.debug.assert(std.mem.eql(u8, data.items, d));
    }
}

Output

Level 1: 1060 -> 72 bytes (6.8%)
Level 3: 1060 -> 72 bytes (6.8%)
Level 6: 1060 -> 72 bytes (6.8%)
Level 9: 1060 -> 72 bytes (6.8%)
Level 15: 1060 -> 72 bytes (6.8%)
Level 19: 1060 -> 72 bytes (6.8%)

Every level gives the same size here, and that is the point of the example rather than a disappointment. The input is 20 copies of one sentence, so the match finder finds the repeat immediately at level 1 and no additional search effort can improve on it. Level controls how hard the encoder looks for matches, and there is nothing left to find. Levels separate on inputs with more structure, or where a level trades speed for a better parse.

Explanation

• compressWithLevel(allocator, src, level) maps the level through getCompressionParameters to a CompressionOptions, then calls the same one-shot path as compressWithOptions.
• Bounds: zstd.minCLevel() is -131072, zstd.maxCLevel() is 22, zstd.defaultCLevel() is 3. Levels are plain i32 across that whole range.
• Each frame is decompressed and compared, so a level that produced a smaller but wrong frame would fail here rather than silently pass.

Run:

zig build run-custom_level