Add slice syntax to sections permalinks config

Fixes #8363

resources/page/permalinks.go

@@ -16,6 +16,7 @@ package page
 import (
 	"fmt"
 	"os"
+	"path"
 	"path/filepath"
 	"regexp"
 	"strconv"
@@ -54,6 +55,13 @@ func (p PermalinkExpander) callback(attr string) (pageToPermaAttribute, bool) {
 		return p.pageToPermalinkDate, true
 	}
 
+	if strings.HasPrefix(attr, "sections[") {
+		fn := p.toSliceFunc(strings.TrimPrefix(attr, "sections"))
+		return func(p Page, s string) (string, error) {
+			return path.Join(fn(p.CurrentSection().SectionsEntries())...), nil
+		}, true
+	}
+
 	return nil, false
 }
 
@@ -112,6 +120,7 @@ func (l PermalinkExpander) parse(patterns map[string]string) (map[string]func(Pa
 
 	for k, pattern := range patterns {
 		k = strings.Trim(k, sectionCutSet)
+
 		if !l.validate(pattern) {
 			return nil, &permalinkExpandError{pattern: pattern, err: errPermalinkIllFormed}
 		}
@@ -165,7 +174,7 @@ func (l PermalinkExpander) parse(patterns map[string]string) (map[string]func(Pa
 // can return a string to go in that position in the page (or an error)
 type pageToPermaAttribute func(Page, string) (string, error)
 
-var attributeRegexp = regexp.MustCompile(`:\w+`)
+var attributeRegexp = regexp.MustCompile(`:\w+(\[.+\])?`)
 
 // validate determines if a PathPattern is well-formed
 func (l PermalinkExpander) validate(pp string) bool {
@@ -263,3 +272,90 @@ func (l PermalinkExpander) pageToPermalinkSection(p Page, _ string) (string, err
 func (l PermalinkExpander) pageToPermalinkSections(p Page, _ string) (string, error) {
 	return p.CurrentSection().SectionsPath(), nil
 }
+
+var (
+	nilSliceFunc = func(s []string) []string {
+		return nil
+	}
+	allSliceFunc = func(s []string) []string {
+		return s
+	}
+)
+
+// toSliceFunc returns a slice func that slices s according to the cut spec.
+// The cut spec must be on form [low:high] (one or both can be omitted),
+// also allowing single slice indices (e.g. [2]) and the special [last] keyword
+// giving the last element of the slice.
+// The returned function will be lenient and not panic in out of bounds situation.
+//
+// The current use case for this is to use parts of the sections path in permalinks.
+func (l PermalinkExpander) toSliceFunc(cut string) func(s []string) []string {
+	cut = strings.ToLower(strings.TrimSpace(cut))
+	if cut == "" {
+		return allSliceFunc
+	}
+
+	if len(cut) < 3 || (cut[0] != '[' || cut[len(cut)-1] != ']') {
+		return nilSliceFunc
+	}
+
+	toNFunc := func(s string, low bool) func(ss []string) int {
+		if s == "" {
+			if low {
+				return func(ss []string) int {
+					return 0
+				}
+			} else {
+				return func(ss []string) int {
+					return len(ss)
+				}
+			}
+		}
+
+		if s == "last" {
+			return func(ss []string) int {
+				return len(ss) - 1
+			}
+		}
+
+		n, _ := strconv.Atoi(s)
+		if n < 0 {
+			n = 0
+		}
+		return func(ss []string) int {
+			// Prevent out of bound situations. It would not make
+			// much sense to panic here.
+			if n > len(ss) {
+				return len(ss)
+			}
+			return n
+		}
+	}
+
+	opsStr := cut[1 : len(cut)-1]
+	opts := strings.Split(opsStr, ":")
+
+	if !strings.Contains(opsStr, ":") {
+		toN := toNFunc(opts[0], true)
+		return func(s []string) []string {
+			if len(s) == 0 {
+				return nil
+			}
+			v := s[toN(s)]
+			if v == "" {
+				return nil
+			}
+			return []string{v}
+		}
+	}
+
+	toN1, toN2 := toNFunc(opts[0], true), toNFunc(opts[1], false)
+
+	return func(s []string) []string {
+		if len(s) == 0 {
+			return nil
+		}
+		return s[toN1(s):toN2(s)]
+	}
+
+}