Add block pattern parsing (#1237)

* Revert "Remove pattern code from PR"

This reverts commit d9296b00cff93e7f0bd0962f012aa98f3ec39915.

* Apply review requests

* Update import

* Rename pattern.zig to Pattern.zig

* Fix compilation errors

* Remove code comment

src/server/command/worldedit/Pattern.zig

@@ -0,0 +1,52 @@
+const std = @import("std");
+
+const main = @import("main");
+const AliasTable = main.utils.AliasTable;
+const Block = main.blocks.Block;
+const ListUnmanaged = main.ListUnmanaged;
+const NeverFailingAllocator = main.heap.NeverFailingAllocator;
+
+blocks: AliasTable(Entry),
+
+const Entry = struct {
+	block: Block,
+	chance: f32,
+};
+
+pub fn initFromString(allocator: NeverFailingAllocator, source: []const u8) !@This() {
+	var specifiers = std.mem.splitScalar(u8, source, ',');
+	var totalWeight: f32 = 0;
+
+	var weightedEntries: ListUnmanaged(struct {block: Block, weight: f32}) = .{};
+	defer weightedEntries.deinit(main.stackAllocator);
+
+	while(specifiers.next()) |specifier| {
+		var iterator = std.mem.splitScalar(u8, specifier, '%');
+
+		var weight: f32 = undefined;
+		var block = main.blocks.parseBlock(iterator.rest());
+
+		const first = iterator.first();
+
+		weight = std.fmt.parseFloat(f32, first) catch blk: {
+			// To distinguish somehow between mistyped numeric values and actual block IDs we check for addon name separator.
+			if(!std.mem.containsAtLeastScalar(u8, first, 1, ':')) return error.PatternSyntaxError;
+			block = main.blocks.parseBlock(first);
+			break :blk 1.0;
+		};
+		totalWeight += weight;
+		weightedEntries.append(main.stackAllocator, .{.block = block, .weight = weight});
+	}
+
+	const entries = allocator.alloc(Entry, weightedEntries.items.len);
+	for(weightedEntries.items, 0..) |entry, i| {
+		entries[i] = .{.block = entry.block, .chance = entry.weight/totalWeight};
+	}
+
+	return .{.blocks = .init(allocator, entries)};
+}
+
+pub fn deinit(self: @This(), allocator: NeverFailingAllocator) void {
+	self.blocks.deinit(allocator);
+	allocator.free(self.blocks.items);
+}