refactor: extract common lexer functions from version parser #1615

Author: rholshausen

core/support/src/main/kotlin/au/com/dius/pact/core/support/Version.kt

@@ -1,5 +1,6 @@
 package au.com.dius.pact.core.support
 
+import au.com.dius.pact.core.support.parsers.StringLexer
 import com.github.michaelbull.result.Err
 import com.github.michaelbull.result.Ok
 import com.github.michaelbull.result.Result
@@ -22,68 +23,52 @@ data class Version(
 
     @JvmStatic
     fun parse(version: String): Result<Version, String> {
-      var buffer = version
-      var index = 0
+      val lexer = StringLexer(version)
 
-      val major = when (val result = parseInt(buffer, index)) {
-        is Ok -> {
-          buffer = result.value.second
-          index = result.value.third
-          result.value.first
-        }
+      val major = when (val result = parseInt(lexer)) {
+        is Ok -> result.value
         is Err -> return result
       }
 
-      when (val dot = parseChar('.', buffer, index)) {
-        is Ok -> {
-          buffer = dot.value.first
-          index = dot.value.second
-        }
-        is Err -> {
-          return dot
-        }
+      val err = parseChar('.', lexer)
+      if (err != null) {
+        return Err(err)
       }
 
-      val minor = when (val result = parseInt(buffer, index)) {
-        is Ok -> {
-          buffer = result.value.second
-          index = result.value.third
-          result.value.first
-        }
+      val minor = when (val result = parseInt(lexer)) {
+        is Ok -> result.value
         is Err -> return result
       }
 
-      val dot = parseChar('.', buffer, index)
       return when {
-        dot is Ok -> {
-          buffer = dot.value.first
-          index = dot.value.second
-          when (val result = parseInt(buffer, index)) {
-            is Ok -> Ok(Version(major, minor, result.value.first))
+        lexer.peekNextChar() == '.' -> {
+          lexer.advance()
+          when (val result = parseInt(lexer)) {
+            is Ok -> if (lexer.empty) {
+              Ok(Version(major, minor, result.value))
+            } else {
+              Err("Unexpected characters '${lexer.remainder}' at index ${lexer.index}")
+            }
             is Err -> result
           }
         }
-        buffer.isEmpty() -> Ok(Version(major, minor))
-        else -> Err("Unexpected character '${buffer[0]}' at index $index")
+        lexer.empty -> Ok(Version(major, minor))
+        else -> Err("Unexpected characters '${lexer.remainder}' at index ${lexer.index}")
       }
     }
 
-    private fun parseChar(c: Char, buffer: String, index: Int): Result<Pair<String, Int>, String> {
-      return when {
-        buffer.isNotEmpty() && buffer[0] == c -> {
-          Ok(buffer.substring(1) to (index + 1))
-        }
-        else -> Err("Was expecting a $c at index $index")
+    private fun parseChar(c: Char, lexer: StringLexer): String? {
+      return when (val ch = lexer.nextChar()) {
+        null -> "Was expecting a '$c' at index ${lexer.index} but got the end of the input"
+        c -> null
+        else -> "Was expecting a '$c' at index ${lexer.index - 1} but got '$ch'"
       }
     }
 
-    private fun parseInt(buffer: String, index: Int): Result<Triple<Int, String, Int>, String> {
-      return when (val result = INT.find(buffer)) {
-        null -> Err("Was expecting an integer at index $index")
-        else -> {
-          val i = result.value.toInt()
-          Ok(Triple(i, buffer.substring(result.value.length), index + result.value.length))
-        }
+    private fun parseInt(lexer: StringLexer): Result<Int, String> {
+      return when (val result = lexer.matchRegex(INT)) {
+        null -> Err("Was expecting an integer at index ${lexer.index}")
+        else -> Ok(result.toInt())
       }
     }
   }

core/support/src/main/kotlin/au/com/dius/pact/core/support/generators/expressions/expressions.kt renamed to core/support/src/main/kotlin/au/com/dius/pact/core/support/generators/expressions/Expressions.kt

@@ -0,0 +1,56 @@
+package au.com.dius.pact.core.support.parsers
+
+class StringLexer(private val buffer: String) {
+  var index = 0
+    private set
+
+  val empty: Boolean
+    get() = index >= buffer.length
+
+  val remainder: String
+    get() = buffer.substring(index)
+
+  fun nextChar(): Char? {
+    val c = peekNextChar()
+    if (c != null) {
+      index++
+    }
+    return c
+  }
+
+  fun peekNextChar(): Char? {
+    return if (empty) {
+      null
+    } else {
+      buffer[index]
+    }
+  }
+
+  fun advance() {
+    advance(1)
+  }
+
+  fun advance(count: Int) {
+    for (i in 0 until count) {
+      index++
+    }
+  }
+
+  fun skipWhitespace() {
+    var next = peekNextChar()
+    while (next != null && Character.isWhitespace(next)) {
+      advance()
+      next = peekNextChar()
+    }
+  }
+
+  fun matchRegex(regex: Regex): String? {
+    return when (val result = regex.find(buffer.substring(index))) {
+      null -> null
+      else -> {
+        index += result.value.length
+        result.value
+      }
+    }
+  }
+}

core/support/src/main/kotlin/au/com/dius/pact/core/support/parsers/StringLexer.kt

@@ -31,12 +31,13 @@ class VersionParserSpec extends Specification {
     version      | error
     ''           | 'Was expecting an integer at index 0'
     'sdsd'       | 'Was expecting an integer at index 0'
-    '0'          | 'Was expecting a . at index 1'
-    '0sass'      | 'Was expecting a . at index 1'
-    '100'        | 'Was expecting a . at index 3'
+    '0'          | "Was expecting a '.' at index 1 but got the end of the input"
+    '0sass'      | "Was expecting a '.' at index 1 but got 's'"
+    '100'        | "Was expecting a '.' at index 3 but got the end of the input"
     '100.'       | 'Was expecting an integer at index 4'
     '100.10.'    | 'Was expecting an integer at index 7'
-    '100.10x'    | "Unexpected character 'x' at index 6"
+    '100.10x'    | "Unexpected characters 'x' at index 6"
     '100.10.sss' | 'Was expecting an integer at index 7'
+    '100.10.1ss' | "Unexpected characters 'ss' at index 8"
   }
 }