Add 2021-03 in Nim

challenges/2021/03-binaryDiagnostic/README.md

@@ -1,2 +1,3 @@
 # [Day 3: Binary Diagnostic](https://adventofcode.com/2021/day/3)
 
+The Nim code is a direct translation of the Python code - see the Python code for descriptive comments.

challenges/2021/03-binaryDiagnostic/nim/challenge.nim

@@ -0,0 +1,78 @@
+import std/sequtils
+import std/strformat
+import std/strutils
+
+proc parse(instr: string): tuple[numbers: seq[int], length: int] =
+    let y = instr.splitLines.filter(proc(x: string): bool = x != "")
+    return (y.map(parseBinInt), y[0].len)
+
+proc getBit(number: int, n: int): int = 
+    return (number shr n) and 0b1
+
+proc analyseBits(numbers: seq[int], n: int): tuple[mostCommon, leastCommon: int] =
+    var
+        zeros = 0
+        ones = 0
+    
+    for _, number in numbers:
+        let sel = getBit(number, n)
+        case sel
+        of 0:
+            zeros += 1
+        of 1:
+            ones += 1
+        else:
+            raise newException(ValueError, fmt"impossible value {sel} reached")
+
+    if zeros > ones:
+        return (0, 1)
+    elif ones > zeros:
+        return (1, 0)
+    return (-1, -1)
+
+proc partOne*(instr: string): int =
+    let (numbers, bitLength) = parse(instr)
+    
+    var gamma, epsilon: string
+
+    for bitNumber in countdown(bitLength-1, 0):
+        let (mostCommon, leastCommon) = analyseBits(numbers, bitNumber)
+        gamma.add($mostCommon)
+        epsilon.add($leastCommon)
+
+    return parseBinInt(gamma) * parseBinInt(epsilon)
+
+proc partTwo*(instr: string): int =
+    let (numbers, bitLength) = parse(instr)
+
+    proc find(inp: seq[int], useMostCommon: bool, n: int = bitLength-1): int =
+        if inp.len == 1:
+            return inp[0]
+
+        let (mostCommon, leastCommon) = analyseBits(inp, n)
+
+        var target: int
+        if mostCommon == -1 or leastCommon == -1:
+            if useMostCommon:
+                target = 1
+            else:
+                target = 0
+        else:
+            if useMostCommon:
+                target = mostCommon
+            else:
+                target = leastCommon
+
+        return find(
+            inp.filter(
+                proc(x: int): bool = getBit(x, n) == target,
+            ),
+            useMostCommon,
+            n-1
+        )
+
+    let
+        o2Rating = find(numbers, true)
+        co2Rating = find(numbers, false)
+
+    return o2Rating * co2Rating

challenges/2021/README.md

@@ -8,10 +8,10 @@ Solutions to the [2021 Advent of Code](https://adventofcode.com/2021).
 
 <!-- PARSE START -->
 
-| Day                                 | Status             | Solutions                                                                    | Notes                        |
-| ----------------------------------- | ------------------ | ---------------------------------------------------------------------------- | ---------------------------- |
-| 01 - Sonar Sweep                    | Complete           | [Python](01-sonarSweep/py), [Go](01-sonarSweep/go), [Nim](01-sonarSweep/nim) |                              |
-| 02 - Dive!                          | Complete           | [Python](02-dive/py), [Go](02-dive/go)                                       |                              |
-| 02 - Binary Diagnostic              | Complete           | [Python](03-binaryDiagnostic/py)                                             | Bit twiddling, my favourite! |
+| Day                                 | Status             | Solutions                                                                    | Notes                                                            |
+| ----------------------------------- | ------------------ | ---------------------------------------------------------------------------- | ---------------------------------------------------------------- |
+| 01 - Sonar Sweep                    | Complete           | [Python](01-sonarSweep/py), [Go](01-sonarSweep/go), [Nim](01-sonarSweep/nim) | Numbers and sliding windows.                                     |
+| 02 - Dive!                          | Complete           | [Python](02-dive/py), [Go](02-dive/go)                                       | Have this set of instructions and do something sensible with it. |
+| 02 - Binary Diagnostic              | Complete           | [Python](03-binaryDiagnostic/py), [Nim](03-binaryDiagnostic/nim)             | Bit twiddling aplenty!                                           |
 
 <!-- PARSE END -->