Added reverse op

pkg/yqlib/doc/operators/headers/reverse.md

@@ -0,0 +1,3 @@
+# Reverse
+
+Reverses the order of the items in an array 

pkg/yqlib/doc/operators/headers/sort.md

@@ -2,4 +2,7 @@
 
 Sorts an array. Use `sort` to sort an array as is, or `sort_by(exp)` to sort by a particular expression (e.g. subfield).
 
+To sort by descending order, pipe the results through the `reverse` operator after sorting.
+
 Note that at this stage, `yq` only sorts scalar fields.
+

pkg/yqlib/doc/operators/reverse.md

@@ -0,0 +1,48 @@
+# Reverse
+
+Reverses the order of the items in an array 
+
+{% hint style="warning" %}
+Note that versions prior to 4.18 require the 'eval/e' command to be specified. 
+
+`yq e <exp> <file>`
+{% endhint %}
+
+## Reverse
+Given a sample.yml file of:
+```yaml
+- 1
+- 2
+- 3
+```
+then
+```bash
+yq 'reverse' sample.yml
+```
+will output
+```yaml
+- 3
+- 2
+- 1
+```
+
+## Sort descending by string field
+Use sort with reverse to sort in descending order.
+
+Given a sample.yml file of:
+```yaml
+- a: banana
+- a: cat
+- a: apple
+```
+then
+```bash
+yq 'sort_by(.a) | reverse' sample.yml
+```
+will output
+```yaml
+- a: cat
+- a: banana
+- a: apple
+```
+

pkg/yqlib/doc/operators/sort.md

@@ -2,8 +2,11 @@
 
 Sorts an array. Use `sort` to sort an array as is, or `sort_by(exp)` to sort by a particular expression (e.g. subfield).
 
+To sort by descending order, pipe the results through the `reverse` operator after sorting.
+
 Note that at this stage, `yq` only sorts scalar fields.
 
+
 {% hint style="warning" %}
 Note that versions prior to 4.18 require the 'eval/e' command to be specified. 
 
@@ -28,6 +31,26 @@ will output
 - a: cat
 ```
 
+## Sort descending by string field
+Use sort with reverse to sort in descending order.
+
+Given a sample.yml file of:
+```yaml
+- a: banana
+- a: cat
+- a: apple
+```
+then
+```bash
+yq 'sort_by(.a) | reverse' sample.yml
+```
+will output
+```yaml
+- a: cat
+- a: banana
+- a: apple
+```
+
 ## Sort array in place
 Given a sample.yml file of:
 ```yaml

pkg/yqlib/expression_tokeniser.go

@@ -416,6 +416,7 @@ func initLexer() (*lex.Lexer, error) {
 
 	lexer.Add([]byte(`sort`), opToken(sortOpType))
 	lexer.Add([]byte(`sort_by`), opToken(sortByOpType))
+	lexer.Add([]byte(`reverse`), opToken(reverseOpType))
 
 	lexer.Add([]byte(`any`), opToken(anyOpType))
 	lexer.Add([]byte(`any_c`), opToken(anyConditionOpType))

pkg/yqlib/lib.go

@@ -123,6 +123,7 @@ var getPathOpType = &operationType{Type: "GET_PATH", NumArgs: 0, Precedence: 50,
 
 var explodeOpType = &operationType{Type: "EXPLODE", NumArgs: 1, Precedence: 50, Handler: explodeOperator}
 var sortByOpType = &operationType{Type: "SORT_BY", NumArgs: 1, Precedence: 50, Handler: sortByOperator}
+var reverseOpType = &operationType{Type: "REVERSE", NumArgs: 0, Precedence: 50, Handler: reverseOperator}
 var sortOpType = &operationType{Type: "SORT", NumArgs: 0, Precedence: 50, Handler: sortOperator}
 var sortKeysOpType = &operationType{Type: "SORT_KEYS", NumArgs: 1, Precedence: 50, Handler: sortKeysOperator}
 var joinStringOpType = &operationType{Type: "JOIN", NumArgs: 1, Precedence: 50, Handler: joinStringOperator}

pkg/yqlib/operator_reverse.go

@@ -0,0 +1,34 @@
+package yqlib
+
+import (
+	"container/list"
+	"fmt"
+
+	yaml "gopkg.in/yaml.v3"
+)
+
+func reverseOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+	results := list.New()
+
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
+		candidate := el.Value.(*CandidateNode)
+
+		candidateNode := unwrapDoc(candidate.Node)
+
+		if candidateNode.Kind != yaml.SequenceNode {
+			return context, fmt.Errorf("node at path [%v] is not an array (it's a %v)", candidate.GetNicePath(), candidate.GetNiceTag())
+		}
+
+		reverseList := &yaml.Node{Kind: yaml.SequenceNode, Tag: "!!seq", Style: candidateNode.Style}
+		reverseList.Content = make([]*yaml.Node, len(candidateNode.Content))
+
+		for i, originalNode := range candidateNode.Content {
+			reverseList.Content[len(candidateNode.Content)-i-1] = originalNode
+		}
+		results.PushBack(candidate.CreateReplacement(reverseList))
+
+	}
+
+	return context.ChildContext(results), nil
+
+}

pkg/yqlib/operator_reverse_test.go

@@ -0,0 +1,54 @@
+package yqlib
+
+import "testing"
+
+var reverseOperatorScenarios = []expressionScenario{
+	{
+		description: "Reverse",
+		document:    "[1, 2, 3]",
+		expression:  `reverse`,
+		expected: []string{
+			"D0, P[], (!!seq)::[3, 2, 1]\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   "[]",
+		expression: `reverse`,
+		expected: []string{
+			"D0, P[], (!!seq)::[]\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   "[1]",
+		expression: `reverse`,
+		expected: []string{
+			"D0, P[], (!!seq)::[1]\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   "[1,2]",
+		expression: `reverse`,
+		expected: []string{
+			"D0, P[], (!!seq)::[2, 1]\n",
+		},
+	},
+	{
+		description:    "Sort descending by string field",
+		subdescription: "Use sort with reverse to sort in descending order.",
+		document:       "[{a: banana},{a: cat},{a: apple}]",
+		expression:     `sort_by(.a) | reverse`,
+		expected: []string{
+			"D0, P[], (!!seq)::[{a: cat}, {a: banana}, {a: apple}]\n",
+		},
+	},
+}
+
+func TestReverseOperatorScenarios(t *testing.T) {
+	for _, tt := range reverseOperatorScenarios {
+		testScenario(t, &tt)
+	}
+	documentOperatorScenarios(t, "reverse", reverseOperatorScenarios)
+}

pkg/yqlib/operator_sort_test.go

@@ -11,6 +11,15 @@ var sortByOperatorScenarios = []expressionScenario{
 			"D0, P[], (!!seq)::[{a: apple}, {a: banana}, {a: cat}]\n",
 		},
 	},
+	{
+		description:    "Sort descending by string field",
+		subdescription: "Use sort with reverse to sort in descending order.",
+		document:       "[{a: banana},{a: cat},{a: apple}]",
+		expression:     `sort_by(.a) | reverse`,
+		expected: []string{
+			"D0, P[], (!!seq)::[{a: cat}, {a: banana}, {a: apple}]\n",
+		},
+	},
 	{
 		description: "Sort array in place",
 		document:    "cool: [{a: banana},{a: cat},{a: apple}]",