refactor: clean up solution to part 1 on day 2

cmd/02/main.go

@@ -1,57 +1,24 @@
 package main
 
 import (
-	"strconv"
 	"strings"
 
+	"github.com/StevanFreeborn/advent-of-code-2025/cmd/02/rnge"
 	"github.com/StevanFreeborn/advent-of-code-2025/internal/file"
 )
 
 const SeparatorCharacter = ","
-const RangeSeparatorCharacter = "-"
-
-type Range struct {
-	Start int64
-	End   int64
-}
 
 func SolvePartOne(filePath string) int64 {
 	input := file.ReadAllText(filePath)
 	strRanges := strings.SplitSeq(input, SeparatorCharacter)
 
-	rngs := []Range{}
-
-	for r := range strRanges {
-		parts := strings.Split(strings.TrimSpace(r), "-")
-		start, _ := strconv.ParseInt(parts[0], 10, 64)
-		end, _ := strconv.ParseInt(parts[1], 10, 64)
-		rng := Range{
-			Start: start,
-			End:   end,
-		}
-		rngs = append(rngs, rng)
-	}
-
 	total := int64(0)
 
-	for _, rng := range rngs {
-		for id := rng.Start; id <= rng.End; id++ {
-			idStr := strconv.FormatInt(id, 10)
-			idLength := len(idStr)
-
-			if idLength%2 != 0 {
-				continue
-			}
-
-			midpoint := idLength / 2
-
-			firstHalf := idStr[:midpoint]
-			secondHalf := idStr[midpoint:]
-
-			if firstHalf != secondHalf {
-				continue
-			}
+	for r := range strRanges {
+		rng := rnge.From(r)
 
+		for id := range rng.InvalidIds() {
 			total += id
 		}
 	}

cmd/02/rnge/rnge.go

@@ -0,0 +1,67 @@
+// Package rnge provides a simple structure to represent a range with a start and end value.
+package rnge
+
+import (
+	"iter"
+	"strconv"
+	"strings"
+)
+
+const RangeSeparatorCharacter = "-"
+
+type Range interface {
+	InvalidIds() iter.Seq[int64]
+	Start() int64
+	End() int64
+}
+
+type rnge struct {
+	start int64
+	end   int64
+}
+
+func From(rngeStr string) Range {
+	parts := strings.Split(strings.TrimSpace(rngeStr), RangeSeparatorCharacter)
+	start, _ := strconv.ParseInt(parts[0], 10, 64)
+	end, _ := strconv.ParseInt(parts[1], 10, 64)
+
+	return rnge{
+		start: start,
+		end:   end,
+	}
+}
+
+func (r rnge) Start() int64 {
+	return r.start
+}
+
+func (r rnge) End() int64 {
+	return r.end
+}
+
+func (r rnge) InvalidIds() iter.Seq[int64] {
+	return func(yield func(int64) bool) {
+		for id := r.start; id <= r.end; id++ {
+			idStr := strconv.FormatInt(id, 10)
+			idLength := len(idStr)
+
+			if idLength%2 != 0 {
+				continue
+			}
+
+			midpoint := idLength / 2
+			firstHalf := idStr[:midpoint]
+			secondHalf := idStr[midpoint:]
+
+			if firstHalf != secondHalf {
+				continue
+			}
+
+			ok := yield(id)
+
+			if !ok {
+				return
+			}
+		}
+	}
+}

cmd/02/rnge/rnge_test.go

@@ -0,0 +1,57 @@
+package rnge_test
+
+import (
+	"slices"
+	"testing"
+
+	"github.com/StevanFreeborn/advent-of-code-2025/cmd/02/rnge"
+)
+
+func TestFrom(t *testing.T) {
+	expectedStart := int64(1000)
+	expectedEnd := int64(2000)
+
+	r := rnge.From("1000-2000")
+
+	if r.Start() != expectedStart {
+		t.Errorf("expected start %d, got %d", expectedStart, r.Start())
+	}
+
+	if r.End() != expectedEnd {
+		t.Errorf("expected end %d, got %d", expectedEnd, r.End())
+	}
+}
+
+func TestStart(t *testing.T) {
+	expectedStart := int64(500)
+
+	r := rnge.From("500-1500")
+
+	if r.Start() != expectedStart {
+		t.Errorf("expected start %d, got %d", expectedStart, r.Start())
+	}
+}
+
+func TestEnd(t *testing.T) {
+	expectedEnd := int64(1500)
+
+	r := rnge.From("500-1500")
+
+	if r.End() != expectedEnd {
+		t.Errorf("expected end %d, got %d", expectedEnd, r.End())
+	}
+}
+
+func TestInvalidIds(t *testing.T) {
+	r := rnge.From("1000-1100")
+	expectedInvalidIds := []int64{1010}
+	resultInvalidIds := []int64{}
+
+	for id := range r.InvalidIds() {
+		resultInvalidIds = append(resultInvalidIds, id)
+	}
+
+	if slices.Equal(resultInvalidIds, expectedInvalidIds) == false {
+		t.Errorf("expected invalid IDs %v, got %v", expectedInvalidIds, resultInvalidIds)
+	}
+}