Day 6

day6/build.zig

@@ -0,0 +1,27 @@
+const std = @import("std");
+
+pub fn build(b: *std.build.Builder) void {
+    // Standard target options allows the person running `zig build` to choose
+    // what target to build for. Here we do not override the defaults, which
+    // means any target is allowed, and the default is native. Other options
+    // for restricting supported target set are available.
+    const target = b.standardTargetOptions(.{});
+
+    // Standard release options allow the person running `zig build` to select
+    // between Debug, ReleaseSafe, ReleaseFast, and ReleaseSmall.
+    const mode = b.standardReleaseOptions();
+
+    const exe = b.addExecutable("day6", "src/main.zig");
+    exe.setTarget(target);
+    exe.setBuildMode(mode);
+    exe.install();
+
+    const run_cmd = exe.run();
+    run_cmd.step.dependOn(b.getInstallStep());
+    if (b.args) |args| {
+        run_cmd.addArgs(args);
+    }
+
+    const run_step = b.step("run", "Run the app");
+    run_step.dependOn(&run_cmd.step);
+}

day6/input

@@ -0,0 +1 @@
+1,1,1,1,1,5,1,1,1,5,1,1,3,1,5,1,4,1,5,1,2,5,1,1,1,1,3,1,4,5,1,1,2,1,1,1,2,4,3,2,1,1,2,1,5,4,4,1,4,1,1,1,4,1,3,1,1,1,2,1,1,1,1,1,1,1,5,4,4,2,4,5,2,1,5,3,1,3,3,1,1,5,4,1,1,3,5,1,1,1,4,4,2,4,1,1,4,1,1,2,1,1,1,2,1,5,2,5,1,1,1,4,1,2,1,1,1,2,2,1,3,1,4,4,1,1,3,1,4,1,1,1,2,5,5,1,4,1,4,4,1,4,1,2,4,1,1,4,1,3,4,4,1,1,5,3,1,1,5,1,3,4,2,1,3,1,3,1,1,1,1,1,1,1,1,1,4,5,1,1,1,1,3,1,1,5,1,1,4,1,1,3,1,1,5,2,1,4,4,1,4,1,2,1,1,1,1,2,1,4,1,1,2,5,1,4,4,1,1,1,4,1,1,1,5,3,1,4,1,4,1,1,3,5,3,5,5,5,1,5,1,1,1,1,1,1,1,1,2,3,3,3,3,4,2,1,1,4,5,3,1,1,5,5,1,1,2,1,4,1,3,5,1,1,1,5,2,2,1,4,2,1,1,4,1,3,1,1,1,3,1,5,1,5,1,1,4,1,2,1

day6/src/main.zig

@@ -0,0 +1,100 @@
+const std = @import("std");
+
+pub fn main() anyerror!void {
+    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
+    defer arena.deinit();
+    const alloc = &arena.allocator;
+
+    // Read our input
+    var f = try std.fs.cwd().openFile("input", .{});
+    defer f.close();
+    var contents = try f.readToEndAlloc(alloc, std.math.maxInt(u32));
+    defer alloc.free(contents);
+    var it = std.mem.tokenize(contents, ",");
+
+    // not unsurprisingly, tracking all the fish individually
+    // runs out of memory. maybe not? I might have misunderstood
+    // an error regarding parsing of a integer with a string
+    // that has a newline in it.
+    // good thing I abandoned this approach, since part 2
+    // clearly is larger than the memory I have available.
+    // var fish = std.ArrayList(i8).init(alloc);
+    // defer fish.deinit();
+
+    // while (it.next()) |fi| {
+    //     try fish.append(try std.fmt.parseInt(i8, fi, 10));
+    // }
+
+    // var n_cycles: u8 = 0;
+    // while (n_cycles < 80) : (n_cycles += 1) {
+    //     var i: usize = 0;
+    //     var len = fish.items.len;
+    //     while (i < len) {
+    //         fish.items[i] -= 1;
+    //         if (fish.items[i] < 0) {
+    //             fish.items[i] = 6;
+    //             try fish.append(8);
+    //         }
+    //     }
+    // }
+
+    // std.log.info("[Part 1] After {} cycles, there are {} fish",
+    //              .{n_cycles, fish.items.len});
+
+    var days = [_] u64 {
+        0, // 0: ready to spawn
+        0, // 1: day 0
+        0, // ...
+        0,
+        0,
+        0,
+        0,
+        0, // 7: day 6
+        0,
+        0, // 9: day 8
+    };
+    while (it.next()) |fi| {
+        var val = try std.fmt.parseInt(u64, fi[0..1], 10);
+        days[val+1] += 1; // our index is val+1, since
+        // we have slot 0 storing currently reproducing
+        // fish
+    }
+
+    var n_cycles: u16 = 0;
+    while (n_cycles < 80) : (n_cycles += 1) {
+        var i: usize = 1;
+        while (i < days.len) : (i += 1) {
+            // Move all fish from i to i-1
+            days[i-1] = days[i];
+        }
+        // Add new fish to days[9]
+        days[9] = days[0];
+        // Move fish from days[0] to days[7]
+        days[7] += days[0];
+        days[0] = 0;
+    }
+    var sum: u64 = 0;
+    for (days) |v| {
+        sum += v;
+    }
+    std.log.info("[Part 1] After {} cycles, there are {} fish",
+                 .{n_cycles, sum});
+    while (n_cycles < 256) : (n_cycles += 1) {
+        var i: usize = 1;
+        while (i < days.len) : (i += 1) {
+            // Move all fish from i to i-1
+            days[i-1] = days[i];
+        }
+        // Add new fish to days[9]
+        days[9] = days[0];
+        // Move fish from days[0] to days[7]
+        days[7] += days[0];
+        days[0] = 0;
+    }
+    sum = 0;
+    for (days) |v| {
+        sum += v;
+    }
+    std.log.info("[Part 2] After {} cycles, there are {} fish",
+                 .{n_cycles, sum});
+}