Add tryIndexFromEnd() function

docs/reference/schemas/config/functions/tryIndexFromEnd.md

@@ -0,0 +1,309 @@
+---
+description: Reference for the 'tryIndexFromEnd' DSC configuration document function
+ms.date:     01/29/2025
+ms.topic:    reference
+title:       tryIndexFromEnd
+---
+
+## Synopsis
+
+Safely retrieves a value from an array by counting backward from the end without
+throwing an error if the index is out of range.
+
+## Syntax
+
+```Syntax
+tryIndexFromEnd(sourceArray, reverseIndex)
+```
+
+## Description
+
+The `tryIndexFromEnd()` function provides a safe way to access array elements by
+counting backward from the end using a one-based index. Unlike standard array
+indexing that might fail with out-of-bounds errors, this function returns `null`
+when the index is invalid or out of range.
+
+This is particularly useful when working with dynamic arrays where the length
+isn't known in advance, or when implementing fallback logic that needs to handle
+missing data gracefully. The function uses a one-based index, meaning `1`
+refers to the last element, `2` to the second-to-last, and so on.
+
+The function returns `null` in the following cases:
+
+- The reverse index is greater than the array length
+- The reverse index is zero or negative
+- The array is empty
+
+## Examples
+
+### Example 1 - Access recent deployment history safely
+
+Use `tryIndexFromEnd()` to access recent deployment records when you're not
+certain how many deployments have occurred. This is useful for rollback
+scenarios where you want to retrieve the previous deployment without causing
+errors if the history is empty or shorter than expected. This example uses
+[`createArray()`][05] to build the deployment history and [`last()`][00] to get
+the current deployment.
+
+```yaml
+# tryIndexFromEnd.example.1.dsc.config.yaml
+$schema: https://aka.ms/dsc/schemas/v3/bundled/config/document.json
+resources:
+- name: Deployment Rollback
+  type: Microsoft.DSC.Debug/Echo
+  properties:
+    output:
+      currentDeployment: "[last(createArray('v1.0.0', 'v1.1.0', 'v1.2.0'))]"
+      previousDeployment: "[tryIndexFromEnd(createArray('v1.0.0', 'v1.1.0', 'v1.2.0'), 2)]"
+      fallbackDeployment: "[tryIndexFromEnd(createArray('v1.0.0', 'v1.1.0', 'v1.2.0'), 10)]"
+```
+
+```bash
+dsc config get --file tryIndexFromEnd.example.1.dsc.config.yaml
+```
+
+```yaml
+results:
+- name: Deployment Rollback
+  type: Microsoft.DSC.Debug/Echo
+  result:
+    actualState:
+      output:
+        currentDeployment: v1.2.0
+        previousDeployment: v1.1.0
+        fallbackDeployment: null
+messages: []
+hadErrors: false
+```
+
+The function returns `v1.1.0` for the second-to-last deployment, and `null` for
+the non-existent 10th-from-last deployment, allowing your configuration to
+handle missing data gracefully.
+
+### Example 2 - Select backup retention with safe defaults
+
+Use `tryIndexFromEnd()` to implement flexible backup retention policies that
+adapt to available backups without failing when fewer backups exist than
+expected. This example retrieves the third-most-recent backup if available. This
+example uses [`parameters()`][06] to reference the backup timestamps array.
+
+```yaml
+# tryIndexFromEnd.example.2.dsc.config.yaml
+$schema: https://aka.ms/dsc/schemas/v3/bundled/config/document.json
+parameters:
+  backupTimestamps:
+    type: array
+    defaultValue:
+      - 20250101
+      - 20250108
+      - 20250115
+      - 20250122
+      - 20250129
+resources:
+- name: Backup Retention
+  type: Microsoft.DSC.Debug/Echo
+  properties:
+    output:
+      backups: "[parameters('backupTimestamps')]"
+      retainAfter: "[tryIndexFromEnd(parameters('backupTimestamps'), 3)]"
+      description: "Retain backups newer than the third-most-recent"
+```
+
+```bash
+dsc config get --file tryIndexFromEnd.example.2.dsc.config.yaml
+```
+
+```yaml
+results:
+- name: Backup Retention
+  type: Microsoft.DSC.Debug/Echo
+  result:
+    actualState:
+      output:
+        backups:
+        - 20250101
+        - 20250108
+        - 20250115
+        - 20250122
+        - 20250129
+        retainAfter: 20250115
+        description: Retain backups newer than the third-most-recent
+messages: []
+hadErrors: false
+```
+
+The function safely returns `20250115` (the third-from-last backup), allowing
+you to implement a retention policy that keeps the three most recent backups.
+
+### Example 3 - Parse log levels from configuration arrays
+
+Use `tryIndexFromEnd()` to access configuration values from arrays of varying
+lengths. This is useful when configuration arrays might have different numbers
+of elements across environments. This example uses [`parameters()`][06] to
+reference the log level arrays.
+
+```yaml
+# tryIndexFromEnd.example.3.dsc.config.yaml
+$schema: https://aka.ms/dsc/schemas/v3/bundled/config/document.json
+parameters:
+  productionLevels:
+    type: array
+    defaultValue: [ERROR, WARN, INFO]
+  devLevels:
+    type: array
+    defaultValue: [ERROR, WARN, INFO, DEBUG, TRACE]
+resources:
+- name: Log Configuration
+  type: Microsoft.DSC.Debug/Echo
+  properties:
+    output:
+      productionLevels: "[parameters('productionLevels')]"
+      devLevels: "[parameters('devLevels')]"
+      prodThirdLevel: "[tryIndexFromEnd(parameters('productionLevels'), 3)]"
+      devThirdLevel: "[tryIndexFromEnd(parameters('devLevels'), 3)]"
+      prodFifthLevel: "[tryIndexFromEnd(parameters('productionLevels'), 5)]"
+```
+
+```bash
+dsc config get --file tryIndexFromEnd.example.3.dsc.config.yaml
+```
+
+```yaml
+results:
+- name: Log Configuration
+  type: Microsoft.DSC.Debug/Echo
+  result:
+    actualState:
+      output:
+        productionLevels:
+        - ERROR
+        - WARN
+        - INFO
+        devLevels:
+        - ERROR
+        - WARN
+        - INFO
+        - DEBUG
+        - TRACE
+        prodThirdLevel: ERROR
+        devThirdLevel: INFO
+        prodFifthLevel: null
+messages: []
+hadErrors: false
+```
+
+The function safely handles arrays of different lengths, returning the
+appropriate log level or `null` without throwing errors.
+
+### Example 4 - Access region-specific configuration with fallback
+
+Use `tryIndexFromEnd()` with [`coalesce()`][02] to implement fallback logic when
+accessing configuration values from arrays that might have different lengths
+across regions. This example shows how to safely access regional endpoints with
+a default fallback. This example uses [`createArray()`][05] to build the
+regional endpoint arrays.
+
+```yaml
+# tryIndexFromEnd.example.4.dsc.config.yaml
+$schema: https://aka.ms/dsc/schemas/v3/bundled/config/document.json
+resources:
+- name: Regional Endpoints
+  type: Microsoft.DSC.Debug/Echo
+  properties:
+    output:
+      primaryRegion: "[createArray('us-east-1', 'us-west-2', 'eu-west-1')]"
+      secondaryRegion: "[createArray('us-west-1')]"
+      preferredPrimary: "[coalesce(tryIndexFromEnd(createArray('us-east-1', 'us-west-2', 'eu-west-1'), 2), 'us-east-1')]"
+      preferredSecondary: "[coalesce(tryIndexFromEnd(createArray('us-west-1'), 2), 'us-west-1')]"
+```
+
+```bash
+dsc config get --file tryIndexFromEnd.example.4.dsc.config.yaml
+```
+
+```yaml
+results:
+- name: Regional Endpoints
+  type: Microsoft.DSC.Debug/Echo
+  result:
+    actualState:
+      output:
+        primaryRegion:
+        - us-east-1
+        - us-west-2
+        - eu-west-1
+        secondaryRegion:
+        - us-west-1
+        preferredPrimary: us-west-2
+        preferredSecondary: us-west-1
+messages: []
+hadErrors: false
+```
+
+By combining `tryIndexFromEnd()` with `coalesce()`, you get robust fallback
+behavior: `preferredPrimary` returns `us-west-2` (the second-to-last region),
+while `preferredSecondary` falls back to the default `us-west-1` when the
+second-to-last element doesn't exist.
+
+## Parameters
+
+### sourceArray
+
+The array to retrieve the element from by counting backward from the end.
+Required.
+
+```yaml
+Type:     array
+Required: true
+Position: 1
+```
+
+### reverseIndex
+
+The one-based index from the end of the array. Must be a positive integer where
+`1` refers to the last element, `2` to the second-to-last, and so on. Required.
+
+```yaml
+Type:     integer
+Required: true
+Position: 2
+Minimum:  1
+```
+
+## Output
+
+Returns the array element at the specified reverse index if the index is valid
+(within array bounds). Returns `null` if the index is out of range, zero,
+negative, or if the array is empty.
+
+The return type matches the type of the element in the array.
+
+```yaml
+Type: any | null
+```
+
+## Errors
+
+The function returns an error in the following cases:
+
+- **Invalid source type**: The first argument is not an array
+- **Invalid index type**: The second argument is not an integer
+
+## Related functions
+
+- [`last()`][00] - Returns the last element of an array (throws error if empty)
+- [`first()`][01] - Returns the first element of an array or character of a string
+- [`coalesce()`][02] - Returns the first non-null value from a list
+- [`equals()`][03] - Compares two values for equality
+- [`not()`][04] - Inverts a boolean value
+- [`createArray()`][05] - Creates an array from provided values
+- [`parameters()`][06] - Returns the value of a specified configuration parameter
+
+<!-- Link reference definitions -->
+[00]: ./last.md
+[01]: ./first.md
+[02]: ./coalesce.md
+[03]: ./equals.md
+[04]: ./not.md
+[05]: ./createArray.md
+[06]: ./parameters.md

dsc/tests/dsc_functions.tests.ps1

@@ -956,6 +956,40 @@ Describe 'tests for function expressions' {
     }
   }
 
+  It 'tryIndexFromEnd() function works for: <expression>' -TestCases @(
+    @{ expression = "[tryIndexFromEnd(createArray('a', 'b', 'c'), 1)]"; expected = 'c' }
+    @{ expression = "[tryIndexFromEnd(createArray('a', 'b', 'c'), 2)]"; expected = 'b' }
+    @{ expression = "[tryIndexFromEnd(createArray('a', 'b', 'c'), 3)]"; expected = 'a' }
+    @{ expression = "[tryIndexFromEnd(createArray('a', 'b', 'c'), 4)]"; expected = $null }
+    @{ expression = "[tryIndexFromEnd(createArray('a', 'b', 'c'), 0)]"; expected = $null }
+    @{ expression = "[tryIndexFromEnd(createArray('a', 'b', 'c'), -1)]"; expected = $null }
+    @{ expression = "[tryIndexFromEnd(createArray('only'), 1)]"; expected = 'only' }
+    @{ expression = "[tryIndexFromEnd(createArray(10, 20, 30, 40), 2)]"; expected = 30 }
+    @{ expression = "[tryIndexFromEnd(createArray(createObject('k', 'v1'), createObject('k', 'v2')), 1)]"; expected = [pscustomobject]@{ k = 'v2' } }
+    @{ expression = "[tryIndexFromEnd(createArray(createArray(1, 2), createArray(3, 4)), 1)]"; expected = @(3, 4) }
+    @{ expression = "[tryIndexFromEnd(createArray(), 1)]"; expected = $null }
+    @{ expression = "[tryIndexFromEnd(createArray('x', 'y'), 1000)]"; expected = $null }
+  ) {
+    param($expression, $expected)
+
+    $config_yaml = @"
+            `$schema: https://aka.ms/dsc/schemas/v3/bundled/config/document.json
+            resources:
+            - name: Echo
+              type: Microsoft.DSC.Debug/Echo
+              properties:
+                output: "$expression"
+"@
+    $out = $config_yaml | dsc config get -f - | ConvertFrom-Json
+    if ($expected -is [pscustomobject]) {
+      ($out.results[0].result.actualState.output | Out-String) | Should -BeExactly ($expected | Out-String)
+    } elseif ($expected -is [array]) {
+      ($out.results[0].result.actualState.output | ConvertTo-Json -Compress) | Should -BeExactly ($expected | ConvertTo-Json -Compress)
+    } else {
+      $out.results[0].result.actualState.output | Should -BeExactly $expected
+    }
+  }
+
   It 'uriComponent function works for: <testInput>' -TestCases @(
     @{ testInput = 'hello world' }
     @{ testInput = 'hello@example.com' }

lib/dsc-lib/locales/en-us.toml

@@ -547,6 +547,12 @@ invoked = "tryGet function"
 invalidKeyType = "Invalid key type, must be a string"
 invalidIndexType = "Invalid index type, must be an integer"
 
+[functions.tryIndexFromEnd]
+description = "Retrieves a value from an array by counting backward from the end. Returns null if the index is out of range."
+invoked = "tryIndexFromEnd function"
+invalidSourceType = "Invalid source type, must be an array"
+invalidIndexType = "Invalid index type, must be an integer"
+
 [functions.union]
 description = "Returns a single array or object with all elements from the parameters"
 invoked = "union function"

lib/dsc-lib/src/functions/mod.rs

@@ -71,6 +71,7 @@ pub mod to_upper;
 pub mod trim;
 pub mod r#true;
 pub mod try_get;
+pub mod try_index_from_end;
 pub mod union;
 pub mod unique_string;
 pub mod uri;
@@ -200,6 +201,7 @@ impl FunctionDispatcher {
             Box::new(trim::Trim{}),
             Box::new(r#true::True{}),
             Box::new(try_get::TryGet{}),
+            Box::new(try_index_from_end::TryIndexFromEnd{}),
             Box::new(union::Union{}),
             Box::new(unique_string::UniqueString{}),
             Box::new(uri::Uri{}),