Shorten error message of string comparison when it is too long

Functions/Assertions/Be.Tests.ps1

@@ -109,6 +109,23 @@ InModuleScope Pester {
         #are not tested here thoroughly, but the behaviour was visually checked and is
         #implicitly tested by using the whole output in the following tests
 
+        It "Shows excerpted error messages correctly" {
+            $expected = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"
+            $actual = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"
+            { $actual | Should Be $expected } | Should Throw "Expected: '...aaaaabbbbb...'"
+        }
+
+        It "Shows excerpted error messages correctly" {
+            $expected = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"
+            $actual = "abb"
+            { $actual | Should Be $expected } | Should Throw "Expected: 'aaaaaaaaaa...'"
+        }
+
+        It "Shows excerpted 'actual values' correctly" {
+            $expected = "aaab"
+            $actual = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"
+            { $actual | Should Be $expected } | Should Throw "But was:  'aaaaaaaaaa...'"
+        }
 
         It "Returns nothing for two identical strings" {
             #this situation should actually never happen, as the code is called

Functions/Assertions/Be.ps1

@@ -167,7 +167,6 @@ function Get-CompareStringMessage {
         }
     }
 
-    [string]$output = $null
     if ($null -ne $differenceIndex) {
         "Expected strings to be the same,$(Format-Because $Because) but they were different."
 
@@ -180,9 +179,11 @@ function Get-CompareStringMessage {
             "String lengths are both $ExpectedValueLength."
             "Strings differ at index $differenceIndex."
         }
-
-        "Expected: '{0}'" -f ( $ExpectedValue | Expand-SpecialCharacters )
-        "But was:  '{0}'" -f ( $actual | Expand-SpecialCharacters )
+        $ellipsis = "..."
+        $excerptSize = 5;
+        $longStringOffset = 0
+        "Expected: '{0}'" -f ( $ExpectedValue | Format-AsExcerpt -startIndex $differenceIndex -excerptSize $excerptSize  -excerptMarker $ellipsis | Expand-SpecialCharacters )
+        "But was:  '{0}'" -f ( $actual | Format-AsExcerpt -startIndex $differenceIndex -excerptSize $excerptSize -excerptMarker $ellipsis | Expand-SpecialCharacters )
 
         $specialCharacterOffset = $null
         if ($differenceIndex -ne 0) {
@@ -193,9 +194,54 @@ function Get-CompareStringMessage {
                     & $SafeCommands['Select-Object'] -ExpandProperty Count)
         }
 
-        '-' * ($differenceIndex + $specialCharacterOffset + 11) + '^'
+        # for excerpted strings, add in an additional length of arrow...
+        $excerptOffset = $ellipsis.Length + $excerptSize
+        if ($differenceIndex -ge $excerptOffset) {
+            $longStringOffset = $excerptOffset
+        }
+
+        '-' * (  $differenceIndex + $specialCharacterOffset + 11 + $longStringOffset) + '^'
     }
 }
+function Format-AsExcerpt {
+    param (
+        [Parameter(Mandatory = $true, ValueFromPipeline = $true, ValueFromPipelineByPropertyName = $true)]
+        [AllowEmptyString()]
+        [string]$InputObject,
+        [Parameter(Mandatory = $true, ValueFromPipelineByPropertyName = $true)]
+        [int]$startIndex,
+        [Parameter(Mandatory = $true, ValueFromPipelineByPropertyName = $true)]
+        [int]$excerptSize,
+        [Parameter(Mandatory = $true, ValueFromPipelineByPropertyName = $true)]
+        [string]$excerptMarker
+    )
+    $InputObjectDisplay=""
+    $displayDifferenceIndex = $startIndex - $excerptSize
+    $maximumStringLength = 40
+    $maximumSubstringLength = $excerptSize * 2
+    $substringLength = $InputObject.Length - $displayDifferenceIndex
+    if ($substringLength -gt $maximumSubstringLength) {
+        $substringLength = $maximumSubstringLength
+    }
+    if ($displayDifferenceIndex + $substringLength -lt $InputObject.Length) {
+        $endExcerptMarker = $excerptMarker
+    }
+    if ($displayDifferenceIndex -lt 0) {
+        $displayDifferenceIndex = 0
+    }
+    if ($InputObject.length -ge $maximumStringLength) {
+        if ($displayDifferenceIndex -ne 0) {
+            $InputObjectDisplay = $excerptMarker
+        }
+        $InputObjectDisplay += $InputObject.Substring($displayDifferenceIndex, $substringLength) + $endExcerptMarker
+    }
+    else {
+        $InputObjectDisplay = $InputObject
+    }
+    $InputObjectDisplay
+}
+
+
 
 function Expand-SpecialCharacters {
     param (
@@ -291,4 +337,3 @@ function ReplaceValueInArray {
         }
     }
 }
-