Solution to 1-2

NOTES.md

@@ -3,6 +3,8 @@ Notes, thoughts, or questions that arose as I implemented the solutions. Hopeful
 # Things I like
 
 * [Continue expressions](https://zig-by-example.com/while)
+* Built in optionals (with `orelse`)
+* Better error-handling than GoLang's (though that bar is set _real_ low). I have only just scratched the surface, though, it looks interestingly powerful - might well be even better than I've realized at this point!
 
 # Things that I've found missing from this language
 
@@ -10,6 +12,7 @@ I mean, Jesus Christ, right now it seems even worse than GoLang.
 
 * [String concatenation](https://old.reddit.com/r/Zig/comments/bfcsul/concatenating_zig_strings/)
 * [String equality](https://nofmal.github.io/zig-with-example/string-handling/#string-equal)
+* Switching on strings
 
 # Questions
 
@@ -83,3 +86,41 @@ test {
 I can fix it by changing the type signature to accept `[]const u8`, but (I think?) that then means that I can't call the function with non-const-length strings - including strings read from files.
 
 [This](https://stackoverflow.com/questions/72736997/how-to-pass-a-c-string-into-a-zig-function-expecting-a-zig-string) link refers to `[]const u8` as a "Zig-style string-slice", but also refers to `[*c]const u8` as a "c_string", so...:shrug:?
+
+## Why can't I iterate over a HashMap?
+
+The following code:
+
+```zig
+const std = @import("std");
+const print = std.debug.print;
+
+pub fn main() !void {
+    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
+    defer _ = gpa.deinit();
+    const allocator = gpa.allocator();
+    var hashMap = std.AutoHashMap(u32, u32).init(allocator);
+
+    try hashMap.put(2, 5);
+    try hashMap.put(1, 35);
+    try hashMap.put(4, 20);
+
+    const iter = hashMap.keyIterator();
+    while (try iter.next()) |key| {
+        print("{}\n", .{key});
+    }
+}
+```
+
+gives:
+
+```
+scratch.zig:15:20: error: expected type '*hash_map.HashMapUnmanaged(u32,u32,hash_map.AutoContext(u32),80).FieldIterator(u32)', found '*const hash_map.HashMapUnmanaged(u32,u32,hash_map.AutoContext(u32),80).FieldIterator(u32)'
+    while (try iter.next()) |key| {
+               ~~~~^~~~~
+scratch.zig:15:20: note: cast discards const qualifier
+/Users/scubbo/zig/zig-macos-x86_64-0.14.0-dev.2362+a47aa9dd9/lib/std/hash_map.zig:894:35: note: parameter type declared here
+                pub fn next(self: *@This()) ?*T {
+```
+
+I _think_ this means that the pointer to the Iterator is a Const-pointer and `.next()` expects a mutable pointer. But, if so - how do we get a mutable pointer from a const? I tried `@ptrCast` but that gave a similar error.

README.md

@@ -3,3 +3,11 @@
 If you - like me - are new to the Zig language, [Ziglings](https://codeberg.org/ziglings/exercises/) seems to be a well-respected entrypoint!
 
 The authoritative source for this code is on [Gitea](https://gitea.scubbo.org/scubbo/advent-of-code-2024). The GitHub version is a [mirror](https://docs.gitea.com/usage/repo-mirror#setting-up-a-push-mirror-from-gitea-to-github). [Self-hosting](https://old.reddit.com/r/selfhosted/) is not only the best way to learn, but also to reduce dependency on untrustworthy corporations.
+
+# Execution
+
+I've tried (in `main.zig`) to make a general-purpose executable that can be passed arguments to determine the function to run (e.g. `zig run main.zig -- 1 2`), but so far no luck - lots of type errors 🙃
+
+So for now, run directly with (e.g.) `zig run solutions/01.zig`, and do the following manual changes:
+* Change `pub fn main() void {...}` in each solution-file to invoke the function you want run.
+* Change `isTestCase` from `true` to `false` when ready to get the real solution.

main.zig

@@ -0,0 +1,48 @@
+const std = @import("std");
+
+// Unfortunately Zig doesn't appear to support dynamic imports - that is, non-literal arguments to `@import` -
+// so we can't do `const module = @import(util.concatString(&.{"solutions/", problem_number, ".zig"}))`
+
+const problem_one = @import("solutions/01.zig");
+
+pub fn main() void {
+    // std.debug.print("There are {d} args:\n", .{std.os.argv.len});
+    // for (std.os.argv) |arg| {
+    //     std.debug.print("  {s}\n", .{arg});
+    // }
+    const args = std.os.argv;
+    const problem_number = parse_input_as_number(args[1]);
+    const sub_problem_number = parse_input_as_number(args[2]);
+    const module = switch (problem_number) {
+        1 => {
+            return problem_one;
+        },
+        else => {
+            unreachable;
+        },
+    };
+    switch (sub_problem_number) {
+        1 => {
+            module.part_one();
+        },
+        2 => {
+            module.part_two();
+        },
+        else => {
+            unreachable;
+        },
+    }
+}
+
+fn parse_input_as_number(input: [*:0]u8) i32 {
+    // We _should_ be able to just use `try std.fmt.parseInt(i32, input)` - but that complains with
+    // `expected type '[]const u8', found '[*:0]u8'`
+    // and I CBA to figure out how to do that conversion when I have the time pressure of solving problems!
+    // That can be for later learning :P
+    var output_value: i32 = 0;
+    for (input) |char| {
+        output_value *= 10;
+        output_value + char;
+    }
+    return output_value;
+}

scratch.zig

@@ -0,0 +1,18 @@
+const std = @import("std");
+const print = std.debug.print;
+
+pub fn main() !void {
+    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
+    defer _ = gpa.deinit();
+    const allocator = gpa.allocator();
+    var hashMap = std.AutoHashMap(u32, u32).init(allocator);
+
+    try hashMap.put(2, 5);
+    try hashMap.put(1, 35);
+    try hashMap.put(4, 20);
+
+    const iter = hashMap.keyIterator();
+    while (try iter.next()) |key| {
+        print("{}\n", .{key});
+    }
+}

solutions/01.zig

@@ -2,9 +2,13 @@ const std = @import("std");
 const print = std.debug.print;
 const util = @import("util.zig");
 
+pub fn main() !void {
+    const output = try part_two();
+    print("{}\n", .{output});
+}
+
 pub fn part_one() !u32 {
     const arrays = try parseInputFileToArrays();
-    std.debug.print("{any}", .{arrays.first});
     const arr1 = arrays.first;
     const arr2 = arrays.second;
 
@@ -30,7 +34,57 @@ pub fn part_one() !u32 {
 }
 
 pub fn part_two() !u32 {
-    return 0;
+    const arrays = try parseInputFileToArrays();
+    const arr1 = arrays.first;
+    const arr2 = arrays.second;
+    print("DEBUG - arr1 is {any}\narr2 is {any}\n", .{arr1, arr2});
+    // Logic:
+    // * Iterate over each array
+    // * Count the number of occurrences
+    // * For each number, increment the sum by (number * count_1 * count_2)
+    // (Would technically be parallelizable if you wanted to get fancy, but JFC I'm not ready for that in Zig yet)
+
+    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
+    defer _ = gpa.deinit();
+    const allocator = gpa.allocator();
+
+    var counts1 = std.AutoHashMap(u32, u32).init(allocator);
+    var counts2 = std.AutoHashMap(u32, u32).init(allocator);
+    // Later, we'll need to iterate over the keys of `counts1` to sum up the final value.
+    // You'd think you could do this with:
+    // ```
+    // const iter = counts1.keyIterator();
+    // while (try iter.next()) |key| {
+    // ...
+    // ```
+    // But that gives an error that the iterator is a const pointer but `while` requires a mutable pointer.
+    // So instead I preserve the encountered keys in this array so that we can then loop over them.
+    // Ridiculous language.
+    var preservedKeys = std.ArrayList(u32).init(allocator);
+    for (arr1) |elem| {
+        if (!counts1.contains(elem)) {
+            try preservedKeys.append(elem);
+            print("Added {} to the preservedKeys list\n", .{elem});
+        }
+        try counts1.put(elem, (counts1.get(elem) orelse 0) + 1);
+        
+    }
+    for (arr2) |elem| {
+        try counts2.put(elem, (counts2.get(elem) orelse 0) + 1);
+    }
+
+    print("Beginning the calculation", .{});
+    var output: u32 = 0;
+    for (preservedKeys.items) |key| {
+        print("Operating on {} - at this point the output value is {}\n", .{key, output});
+        const count1 = (counts1.get(key) orelse 0);
+        const count2 = (counts2.get(key) orelse 0);
+        const increment = key * count1 * count2;
+        print("The increment for key {} is {} (calculated based on counts {} and {})\n", .{key, increment, count1, count2});
+        output += increment;
+    }
+    return output;
+
 }
 
 fn parseInputFileToArrays() !struct { first: []u32, second: []u32 } {
@@ -118,6 +172,6 @@ test "part one" {
     try expect(try part_one() == 11);
 }
 
-// test "part two" {
-//     try expect(try part_two() == 31);
-// }
+test "part two" {
+    try expect(try part_two() == 31);
+}