From 79b9d5174d89cdff5fe1fa4a4537dcd0d4db0b8e Mon Sep 17 00:00:00 2001
From: Maxim Lebedev <git@toby3d.me>
Date: Fri, 10 Nov 2023 09:11:12 +0600
Subject: [PATCH] :sparkles: Created collections functions namespace for
 templates

---
 .../templateutil/collections/collections.go   | 96 +++++++++++++++++++
 .../collections/collections_test.go           | 39 ++++++++
 internal/templateutil/templateutil.go         | 11 +++
 3 files changed, 146 insertions(+)
 create mode 100644 internal/templateutil/collections/collections.go
 create mode 100644 internal/templateutil/collections/collections_test.go

diff --git a/internal/templateutil/collections/collections.go b/internal/templateutil/collections/collections.go
new file mode 100644
index 0000000..9ce621f
--- /dev/null
+++ b/internal/templateutil/collections/collections.go
@@ -0,0 +1,96 @@
+package collections
+
+import (
+	"errors"
+)
+
+type (
+	Namespace struct{}
+
+	seq struct {
+		first     int
+		increment int
+		last      int
+	}
+)
+
+func New() *Namespace {
+	return &Namespace{}
+}
+
+var (
+	ErrSeqFew    error = errors.New("seq must have at least one argument")
+	ErrSeqMany   error = errors.New("seq must have no more than three arguments")
+	ErrSeqZero   error = errors.New("seq increment must be not zero")
+	ErrSeqLess   error = errors.New("seq increment must be less than zero")
+	ErrSeqMore   error = errors.New("seq increment must be more than zero")
+	ErrIndexLess error = errors.New("index is less than zero")
+	ErrIndexMore error = errors.New("index is more than provided collection size")
+)
+
+func (ns *Namespace) Slice(items ...any) []any {
+	return items
+}
+
+func (ns *Namespace) Seq(args ...int) ([]int, error) {
+	if len(args) < 1 {
+		return nil, ErrSeqFew
+	}
+
+	if 3 < len(args) {
+		return nil, ErrSeqMany
+	}
+
+	opts := seq{
+		first:     1,
+		increment: 1,
+		last:      args[len(args)-1],
+	}
+
+	switch len(args) {
+	case 3: // NOTE(toby3d): first, increment, last
+		opts.increment = args[1]
+
+		switch {
+		case opts.increment == 0:
+			return nil, ErrSeqZero
+		case opts.first < opts.last && opts.increment < 0:
+			return nil, ErrSeqMore
+		case opts.last < opts.first && 0 < opts.increment:
+			return nil, ErrSeqMore
+		}
+
+		fallthrough
+	case 2: // NOTE(toby3d): first, last
+		opts.first = args[0]
+	case 1: // NOTE(toby3d): last
+		if opts.last < 0 {
+			opts.first = opts.last
+			opts.last = -1
+		}
+	}
+
+	out := make([]int, 0)
+
+	if opts.first < opts.last {
+		for i := opts.first; i <= opts.last; i += opts.increment {
+			out = append(out, i)
+		}
+	} else {
+		for i := opts.first; opts.last <= i; i -= opts.increment {
+			out = append(out, i)
+		}
+	}
+
+	return out, nil
+}
+
+func (ns *Namespace) Index(collection []any, index int) (any, error) {
+	if index < 0 {
+		return nil, ErrIndexLess
+	} else if len(collection) < index {
+		return nil, ErrIndexMore
+	}
+
+	return collection[index], nil
+}
diff --git a/internal/templateutil/collections/collections_test.go b/internal/templateutil/collections/collections_test.go
new file mode 100644
index 0000000..109fb60
--- /dev/null
+++ b/internal/templateutil/collections/collections_test.go
@@ -0,0 +1,39 @@
+package collections_test
+
+import (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+
+	"source.toby3d.me/toby3d/home/internal/templateutil/collections"
+)
+
+func TestNamespace_Seq(t *testing.T) {
+	t.Parallel()
+
+	for name, tc := range map[string]struct {
+		input  []int
+		expect []int
+	}{
+		"incremental":     {[]int{3}, []int{1, 2, 3}},
+		"decremental":     {[]int{-3}, []int{-3, -2, -1}},
+		"stepped":         {[]int{1, 2, 4}, []int{1, 3}},
+		"positive-region": {[]int{1, 4}, []int{1, 2, 3, 4}},
+		"negative-region": {[]int{1, -2}, []int{1, 0, -1, -2}},
+	} {
+		name, tc := name, tc
+
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			actual, err := collections.New().Seq(tc.input...)
+			if err != nil {
+				t.Error(err)
+			}
+
+			if diff := cmp.Diff(tc.expect, actual); diff != "" {
+				t.Error(diff)
+			}
+		})
+	}
+}
diff --git a/internal/templateutil/templateutil.go b/internal/templateutil/templateutil.go
index 115a75a..3477c22 100644
--- a/internal/templateutil/templateutil.go
+++ b/internal/templateutil/templateutil.go
@@ -6,6 +6,7 @@ import (
 	"io/fs"
 
 	"source.toby3d.me/toby3d/home/internal/site"
+	"source.toby3d.me/toby3d/home/internal/templateutil/collections"
 	"source.toby3d.me/toby3d/home/internal/templateutil/partials"
 	"source.toby3d.me/toby3d/home/internal/templateutil/safe"
 	"source.toby3d.me/toby3d/home/internal/templateutil/strings"
@@ -69,6 +70,16 @@ func New(dir fs.FS, siter site.UseCase) (template.FuncMap, error) {
 			"markdownify": transformNamespace.Markdownify,
 		},
 	})
+	collectionsNamespace := collections.New()
+	funcs = append(funcs, Function{
+		Name:    "collections",
+		Handler: func(v ...any) any { return collectionsNamespace },
+		Methods: template.FuncMap{
+			"slice": collectionsNamespace.Slice,
+			"seq":   collectionsNamespace.Seq,
+			"index": collectionsNamespace.Index,
+		},
+	})
 
 	for _, f := range funcs {
 		funcMap[f.Name] = f.Handler