add std.AutoHashMap benchmark

see #2

benchmarks/manifest.json

@@ -50,5 +50,13 @@
     "dir": "arena-allocator",
     "mainPath": "main.zig",
     "baselinePath": "main.zig"
+  },
+  "std-hash-map": {
+    "description": "std.AutoHashMap - Project Euler 14",
+    "kind": "zig-bench",
+    "baseline": "feade9ef0010b1b47d7216e786ed964d09612c2b",
+    "dir": "std-hash-map",
+    "mainPath": "main.zig",
+    "baselinePath": "main.zig"
   }
 }

benchmarks/std-hash-map/.gitignore

@@ -0,0 +1,2 @@
+run
+run.o

benchmarks/std-hash-map/main.zig

@@ -0,0 +1,44 @@
+const std = @import("std");
+const bench = @import("root");
+
+pub fn setup(gpa: *std.mem.Allocator, options: *bench.Options) !void { }
+
+pub fn run(gpa: *std.mem.Allocator, context: void) !void {
+    var cache = std.AutoHashMap(u64, u64).init(gpa);
+    defer cache.deinit();
+
+    try cache.ensureCapacity(2000000);
+
+    var x: u64 = 0;
+    var maxx: u64 = 0;
+    var maxl: u64 = 0;
+    while (x < 1000000) : (x += 1) {
+        const l = length(&cache, x);
+        if (l > maxl) {
+            maxl = l;
+            maxx = x;
+        }
+    }
+    if (maxx != 837799) @panic("bad maxx");
+    if (maxl != 524) @panic("bad maxl");
+}
+
+fn step(x: u64) u64 {
+    if (x & 1 > 0) {
+        return 3 * x + 1;
+    } else {
+        return x / 2;
+    }
+}
+
+fn length(cache: *std.AutoHashMap(u64, u64), x: u64) u64 {
+    if (x <= 1) return 0;
+    if (cache.getValue(x)) |e| {
+        return e;
+    } else {
+        const next = step(x);
+        const len = 1 + length(cache, next);
+        cache.putAssumeCapacityNoClobber(x, len);
+        return len;
+    }
+}