feat: add TakeWhile and DropWhile functions (#42)

- Add TakeWhile: returns elements while predicate is true
- Add DropWhile: drops elements while predicate is true
- Comprehensive tests including edge cases
- Benchmarks included

Resolves Issue 14

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

Author: rjNemo

dropwhile.go

@@ -0,0 +1,15 @@
+package underscore
+
+// DropWhile drops elements from the beginning of the slice while the predicate returns true.
+// It returns the remaining elements starting from the first element where the predicate returns false.
+func DropWhile[T any](values []T, predicate func(T) bool) []T {
+	for i, v := range values {
+		if !predicate(v) {
+			res := make([]T, len(values)-i)
+			copy(res, values[i:])
+			return res
+		}
+	}
+	// All elements satisfy predicate, return empty slice
+	return []T{}
+}

dropwhile_test.go

@@ -0,0 +1,55 @@
+package underscore_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	u "github.com/rjNemo/underscore"
+)
+
+func TestDropWhile(t *testing.T) {
+	nums := []int{1, 2, 3, 4, 5, 6, 7, 8, 9}
+	result := u.DropWhile(nums, func(n int) bool { return n < 5 })
+	assert.Equal(t, []int{5, 6, 7, 8, 9}, result)
+}
+
+func TestDropWhileEmpty(t *testing.T) {
+	result := u.DropWhile([]int{}, func(n int) bool { return n < 5 })
+	assert.Equal(t, []int{}, result)
+}
+
+func TestDropWhileNoneMatch(t *testing.T) {
+	nums := []int{5, 6, 7, 8, 9}
+	result := u.DropWhile(nums, func(n int) bool { return n < 5 })
+	assert.Equal(t, []int{5, 6, 7, 8, 9}, result)
+}
+
+func TestDropWhileAllMatch(t *testing.T) {
+	nums := []int{1, 2, 3, 4}
+	result := u.DropWhile(nums, func(n int) bool { return n < 10 })
+	assert.Equal(t, []int{}, result)
+}
+
+func TestDropWhileSingleElement(t *testing.T) {
+	result := u.DropWhile([]int{5}, func(n int) bool { return n < 10 })
+	assert.Equal(t, []int{}, result)
+}
+
+func TestDropWhileStrings(t *testing.T) {
+	words := []string{"apple", "banana", "cherry", "date"}
+	result := u.DropWhile(words, func(s string) bool { return len(s) < 6 })
+	assert.Equal(t, []string{"banana", "cherry", "date"}, result)
+}
+
+func BenchmarkDropWhile(b *testing.B) {
+	nums := make([]int, 1000)
+	for i := range nums {
+		nums[i] = i
+	}
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		u.DropWhile(nums, func(n int) bool { return n < 500 })
+	}
+}

takewhile.go

@@ -0,0 +1,17 @@
+package underscore
+
+// TakeWhile returns elements from the beginning of the slice while the predicate returns true.
+// It stops at the first element where the predicate returns false.
+func TakeWhile[T any](values []T, predicate func(T) bool) []T {
+	for i, v := range values {
+		if !predicate(v) {
+			res := make([]T, i)
+			copy(res, values[:i])
+			return res
+		}
+	}
+	// All elements satisfy predicate
+	res := make([]T, len(values))
+	copy(res, values)
+	return res
+}

takewhile_test.go

@@ -0,0 +1,55 @@
+package underscore_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	u "github.com/rjNemo/underscore"
+)
+
+func TestTakeWhile(t *testing.T) {
+	nums := []int{1, 2, 3, 4, 5, 6, 7, 8, 9}
+	result := u.TakeWhile(nums, func(n int) bool { return n < 5 })
+	assert.Equal(t, []int{1, 2, 3, 4}, result)
+}
+
+func TestTakeWhileEmpty(t *testing.T) {
+	result := u.TakeWhile([]int{}, func(n int) bool { return n < 5 })
+	assert.Equal(t, []int{}, result)
+}
+
+func TestTakeWhileNoneMatch(t *testing.T) {
+	nums := []int{5, 6, 7, 8, 9}
+	result := u.TakeWhile(nums, func(n int) bool { return n < 5 })
+	assert.Equal(t, []int{}, result)
+}
+
+func TestTakeWhileAllMatch(t *testing.T) {
+	nums := []int{1, 2, 3, 4}
+	result := u.TakeWhile(nums, func(n int) bool { return n < 10 })
+	assert.Equal(t, []int{1, 2, 3, 4}, result)
+}
+
+func TestTakeWhileSingleElement(t *testing.T) {
+	result := u.TakeWhile([]int{5}, func(n int) bool { return n < 10 })
+	assert.Equal(t, []int{5}, result)
+}
+
+func TestTakeWhileStrings(t *testing.T) {
+	words := []string{"apple", "banana", "cherry", "date"}
+	result := u.TakeWhile(words, func(s string) bool { return len(s) < 6 })
+	assert.Equal(t, []string{"apple"}, result)
+}
+
+func BenchmarkTakeWhile(b *testing.B) {
+	nums := make([]int, 1000)
+	for i := range nums {
+		nums[i] = i
+	}
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		u.TakeWhile(nums, func(n int) bool { return n < 500 })
+	}
+}