Added first part of the third day in zig

2021/03/first.zig

@@ -0,0 +1,56 @@
+const std = @import("std");
+
+const input = @embedFile("input");
+
+pub fn main() anyerror!void {
+    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
+    defer arena.deinit();
+
+    const allocator = arena.allocator();
+
+    const result = try solve(input, 12, allocator);
+    try std.io.getStdOut().writer().print("{}\n", .{result});
+}
+
+fn solve(puzzle: []const u8, width: u8, allocator: std.mem.Allocator) !u64 {
+    var it = std.mem.tokenize(u8, puzzle, "\n");
+    var bits = std.ArrayList(u64).init(allocator);
+    defer bits.deinit();
+
+    if (it.next()) |line| {
+        for (line) |c| {
+            var v: u8 = 0;
+            if (c == '1') {
+                v = 1;
+            }
+            try bits.append(v);
+        }
+    }
+    var len: usize = 1;
+    while (it.next()) |line| : (len += 1) {
+        for (line) |c, i| {
+            if (c == '1') {
+                bits.items[i] += 1;
+            }
+        }
+    }
+
+    var gamma: u64 = 0;
+    var epsilon: u64 = 0;
+    var i: usize = 0;
+    while (i < width) : (i += 1) {
+        gamma <<= 1;
+        epsilon <<= 1;
+        if (bits.items[i] > len / 2) {
+            gamma += 1;
+        } else {
+            epsilon += 1;
+        }
+    }
+    return gamma * epsilon;
+}
+
+test "solve" {
+    const example = @embedFile("example");
+    try std.testing.expectEqual(solve(example, 5, std.testing.allocator), 198);
+}