feat: Day 3

puzzles/03_1/main.rb

@@ -0,0 +1,34 @@
+def run(f)
+    len = 0
+    bits = []
+
+    f.each do |line|
+        if len == 0
+            # Skip the trailing newline character and convert to 0-based index
+            len = line.length() - 2
+            0.upto(len) { |n| bits[n] = [0, 0] }
+        end
+
+        len.downto(0) { |n|
+            digit = Integer(line[n])
+            bits[n][digit] += 1
+        }
+    end
+
+    gamma = 0
+    epsilon = 0
+
+    len.downto(0) { |n|
+        # Since we iterated from left to right on the input
+        # we need to do the same when building the integers
+        width = len - n
+
+        if bits[n][0] > bits[n][1]
+            gamma += 1 << width
+        else
+            epsilon += 1 << width
+        end
+    }
+
+    return gamma * epsilon
+end

puzzles/03_2/main.rb

@@ -0,0 +1,42 @@
+def find(lines, index, search)
+    if index > (lines.first.length() - 2)
+        raise "Invalid index"
+    end
+
+    bits = [0, 0]
+
+    # Count the occurrences of each digit at the index
+    lines.each do |line|
+        digit = Integer(line[index])
+        bits[digit] += 1
+    end
+
+    # Not the most expressive way to do this, but should get the job done
+    if search == 1
+        # Return most common value, or 1 on a tie
+        target = bits[0] > bits[1] ? 0 : 1
+    else
+        # Return least common value, or 0 on a tie
+        target = bits[0] <= bits[1] ? 0 : 1
+    end
+
+    # Filter lines to only the ones that have the target value at `index`
+    lines = lines.filter { |line| line[index] == target.to_s }
+
+    # If there's only one left, we found our value.
+    # Otherwise, continue filtering at the next index
+    if lines.length() == 1
+        return lines.first
+    else
+        return find(lines, index + 1, search)
+    end
+end
+
+def run(f)
+    lines = f.readlines()
+    # This should filter down to a single number, which we need to convert from a base 2 bit string
+    oxygen = find(lines, 0, 1).to_i(2)
+    co2 = find(lines, 0, 0).to_i(2)
+
+    return oxygen * co2
+end