day 6, part 2

day06-hard/build.zig

@@ -0,0 +1,70 @@
+const std = @import("std");
+
+// Although this function looks imperative, note that its job is to
+// declaratively construct a build graph that will be executed by an external
+// runner.
+pub fn build(b: *std.Build) 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 optimization options allow the person running `zig build` to select
+    // between Debug, ReleaseSafe, ReleaseFast, and ReleaseSmall. Here we do not
+    // set a preferred release mode, allowing the user to decide how to optimize.
+    const optimize = b.standardOptimizeOption(.{});
+
+    const exe = b.addExecutable(.{
+        .name = "day06-hard",
+        // In this case the main source file is merely a path, however, in more
+        // complicated build scripts, this could be a generated file.
+        .root_source_file = .{ .path = "src/main.zig" },
+        .target = target,
+        .optimize = optimize,
+    });
+
+    // This declares intent for the executable to be installed into the
+    // standard location when the user invokes the "install" step (the default
+    // step when running `zig build`).
+    b.installArtifact(exe);
+
+    // This *creates* a Run step in the build graph, to be executed when another
+    // step is evaluated that depends on it. The next line below will establish
+    // such a dependency.
+    const run_cmd = b.addRunArtifact(exe);
+
+    // By making the run step depend on the install step, it will be run from the
+    // installation directory rather than directly from within the cache directory.
+    // This is not necessary, however, if the application depends on other installed
+    // files, this ensures they will be present and in the expected location.
+    run_cmd.step.dependOn(b.getInstallStep());
+
+    // This allows the user to pass arguments to the application in the build
+    // command itself, like this: `zig build run -- arg1 arg2 etc`
+    if (b.args) |args| {
+        run_cmd.addArgs(args);
+    }
+
+    // This creates a build step. It will be visible in the `zig build --help` menu,
+    // and can be selected like this: `zig build run`
+    // This will evaluate the `run` step rather than the default, which is "install".
+    const run_step = b.step("run", "Run the app");
+    run_step.dependOn(&run_cmd.step);
+
+    // Creates a step for unit testing. This only builds the test executable
+    // but does not run it.
+    const unit_tests = b.addTest(.{
+        .root_source_file = .{ .path = "src/main.zig" },
+        .target = target,
+        .optimize = optimize,
+    });
+
+    const run_unit_tests = b.addRunArtifact(unit_tests);
+
+    // Similar to creating the run step earlier, this exposes a `test` step to
+    // the `zig build --help` menu, providing a way for the user to request
+    // running the unit tests.
+    const test_step = b.step("test", "Run unit tests");
+    test_step.dependOn(&run_unit_tests.step);
+}

day06-hard/sample.in

@@ -0,0 +1,2 @@
+Time:      7  15   30
+Distance:  9  40  200

day06-hard/src/main.zig

@@ -0,0 +1,53 @@
+const std = @import("std");
+
+fn addDigit(result: anytype, digit: u8) void {
+    result.* = (result.* * 10) + (digit - '0');
+}
+
+fn parseLine(line: []const u8) u64 {
+    var index: usize = 9;
+    while (line[index] == ' ') : (index += 1) {}
+
+    var result: u64 = 0;
+    while (index < line.len) : (index += 1) {
+        if (line[index] != ' ') {
+            addDigit(&result, line[index]);
+        }
+    }
+
+    return result;
+}
+
+fn solve(time: u64, distance: u64) u64 {
+    // Boundaries:
+    // x * (time - x) = distance
+    // x^2 - time*x + distance = 0
+    // x = (time/2) +- sqrt((time/2)^2 - 4*distance)
+
+    //std.debug.print("computing for time = {d} distance = {d}\n", .{ time, distance });
+    const deviationx2 = std.math.sqrt(time * time - 4 * distance);
+
+    var min = ((time - deviationx2) / 2) - 5;
+    var max = ((time + deviationx2) / 2) - 5;
+
+    while (min * (time - min) <= distance) : (min += 1) {}
+    while (max * (time - max) > distance) : (max += 1) {}
+
+    //std.debug.print("min = {d}, max = {d}\n", .{ min, max });
+
+    return max - min;
+}
+
+pub fn main() !void {
+    const stdout = std.io.getStdOut().writer();
+
+    const raw_in = std.io.getStdIn();
+    var buffered_reader = std.io.bufferedReader(raw_in.reader());
+    var reader = buffered_reader.reader();
+
+    var line_buffer: [1000]u8 = undefined;
+    var times = parseLine((try reader.readUntilDelimiterOrEof(&line_buffer, '\n')).?);
+    var distances = parseLine((try reader.readUntilDelimiterOrEof(&line_buffer, '\n')).?);
+
+    try stdout.print("{d}\n", .{solve(times, distances)});
+}