fix(sampler): introduce minLength support for string pattern format sample

packages/openapi-sampler/src/samplers/StringSampler.ts

@@ -30,10 +30,10 @@ export class StringSampler implements Sampler {
     'relative-json-pointer': () => '1/relative/json/pointer',
     'regex': () => '/regex/',
     'pattern': (
-      _min: number,
+      min: number,
       max: number,
       { pattern }: { pattern: string | RegExp }
-    ) => this.patternSample(pattern, max),
+    ) => this.patternSample(pattern, min, max),
     'default': (min: number, max: number) =>
       this.adjustLength('lorem', min, max)
   };
@@ -47,9 +47,34 @@ export class StringSampler implements Sampler {
     return this.checkLength(sampler(min || 0, max, schema), format, min, max);
   }
 
-  private patternSample(pattern: string | RegExp, max?: number): string {
+  private patternSample(
+    pattern: string | RegExp,
+    min?: number,
+    max?: number
+  ): string {
     const randExp = new RandExp(pattern);
 
+    if (min) {
+      // ADHOC: make a probe for regex using min quantifier value
+      // e.g. ^[a]+[b]+$ expect 'ab', ^[a-z]*$ expect ''
+
+      randExp.max = 0;
+      randExp.randInt = (a, _) => a;
+
+      const result = randExp.gen();
+
+      if (result.length >= min) {
+        return result;
+      }
+
+      // ADHOC: fallback for failed min quantifier probe with doubled min length
+
+      randExp.max = 2 * min;
+      randExp.randInt = (a, b) => Math.floor((a + b) / 2);
+
+      return this.adjustMaxLength(randExp.gen(), max);
+    }
+
     randExp.max = max ?? randExp.max;
     randExp.randInt = (a, b) => Math.floor((a + b) / 2);
 
@@ -95,4 +120,8 @@ export class StringSampler implements Sampler {
           Math.min(Math.max(sample.length, minLength), maxLength)
         );
   }
+
+  private adjustMaxLength(sample: string, max?: number): string {
+    return max && sample.length >= max ? sample.substring(0, max) : sample;
+  }
 }

packages/openapi-sampler/tests/string.spec.ts

@@ -60,6 +60,73 @@ describe('StringSampler', () => {
       },
       expected: 'EEEEE44444'
     },
+    {
+      input: {
+        type: 'string',
+        format: 'pattern',
+        maxLength: 4,
+        minLength: 4,
+        pattern: '^[0-9]+$'
+      },
+      expected: '4444'
+    },
+    {
+      input: {
+        type: 'string',
+        format: 'pattern',
+        minLength: 5,
+        maxLength: 6,
+        pattern: '^[0-9]+$'
+      },
+      expected: '444444'
+    },
+    {
+      input: {
+        type: 'string',
+        format: 'pattern',
+        minLength: 5,
+        maxLength: 100,
+        pattern: '^[0-9]+$'
+      },
+      expected: '444444'
+    },
+    {
+      input: {
+        type: 'string',
+        format: 'pattern',
+        minLength: 4,
+        pattern: '^[0-9]+$'
+      },
+      expected: '44444'
+    },
+    {
+      input: {
+        type: 'string',
+        format: 'pattern',
+        minLength: 4,
+        pattern: '^[0-9]{4,5}$'
+      },
+      expected: '0000'
+    },
+    {
+      input: {
+        type: 'string',
+        format: 'pattern',
+        minLength: 5,
+        pattern: '^[0-9]{3,5}[A-Z]{3}$'
+      },
+      expected: '000AAA'
+    },
+    {
+      input: {
+        type: 'string',
+        format: 'pattern',
+        maxLength: 4,
+        minLength: 4,
+        pattern: "[^<>%=']*$"
+      },
+      expected: 'QQQQ'
+    },
     {
       input: {
         type: 'string',
@@ -69,6 +136,26 @@ describe('StringSampler', () => {
       },
       expected: ''
     },
+    {
+      input: {
+        type: 'string',
+        format: 'pattern',
+        pattern: '^[a]+[b]+$',
+        maxLength: 2,
+        minLength: 2
+      },
+      expected: 'ab'
+    },
+    {
+      input: {
+        type: 'string',
+        format: 'pattern',
+        pattern: '^[a]{2,4}[b]{2,4}$',
+        maxLength: 4,
+        minLength: 4
+      },
+      expected: 'aabb'
+    },
     {
       input: {
         type: 'string',
@@ -266,6 +353,26 @@ describe('StringSampler', () => {
       expected:
         'Sample string cannot be generated by boundaries: maxLength=5, format=pattern'
     },
+    {
+      input: {
+        minLength: 5,
+        pattern: '^[0-9]{3}',
+        type: 'string'
+      },
+      expected:
+        'Sample string cannot be generated by boundaries: minLength=5, format=pattern'
+    },
+    {
+      input: {
+        type: 'string',
+        format: 'pattern',
+        pattern: '^[a]+[b]+$',
+        maxLength: 1,
+        minLength: 1
+      },
+      expected:
+        'Sample string cannot be generated by boundaries: minLength=1, maxLength=1, format=pattern'
+    },
     {
       input: {
         type: 'string',