Add CommentSpacingRule for end of line comment padding (pinterest#198)

- Should add spaces before end of line comments if they are not at the beginning of a line, and add spaces after the double slash (//) if not present

ktlint-ruleset-standard/src/main/kotlin/com/github/shyiko/ktlint/ruleset/standard/CommentSpacingRule.kt

@@ -0,0 +1,34 @@
+package com.github.shyiko.ktlint.ruleset.standard
+
+import com.github.shyiko.ktlint.core.Rule
+import org.jetbrains.kotlin.com.intellij.lang.ASTNode
+import org.jetbrains.kotlin.com.intellij.psi.PsiComment
+import org.jetbrains.kotlin.com.intellij.psi.PsiWhiteSpace
+import org.jetbrains.kotlin.com.intellij.psi.impl.source.tree.LeafPsiElement
+import org.jetbrains.kotlin.com.intellij.psi.impl.source.tree.PsiWhiteSpaceImpl
+import org.jetbrains.kotlin.com.intellij.psi.util.PsiTreeUtil
+
+class CommentSpacingRule : Rule("comment-spacing") {
+
+    override fun visit(
+        node: ASTNode,
+        autoCorrect: Boolean,
+        emit: (offset: Int, errorMessage: String, canBeAutoCorrected: Boolean) -> Unit
+    ) {
+        if (node is LeafPsiElement && node is PsiComment && node.getText().startsWith("//")) {
+            val prevLeaf = PsiTreeUtil.prevLeaf(node)
+            if (prevLeaf !is PsiWhiteSpace && prevLeaf is LeafPsiElement) {
+                emit(node.startOffset, "Missing space before end of line comment", true)
+                if (autoCorrect) {
+                    node.rawInsertBeforeMe(PsiWhiteSpaceImpl(" "))
+                }
+            }
+            if (!node.getText().startsWith("// ")) {
+                emit(node.startOffset, "Missing space after double slash in end of line comment", true)
+                if (autoCorrect) {
+                    node.rawReplaceWithText("// " + node.getText().removePrefix("//"))
+                }
+            }
+        }
+    }
+}

ktlint-ruleset-standard/src/main/kotlin/com/github/shyiko/ktlint/ruleset/standard/StandardRuleSetProvider.kt

@@ -7,6 +7,7 @@ class StandardRuleSetProvider : RuleSetProvider {
 
     override fun get(): RuleSet = RuleSet("standard",
         ChainWrappingRule(),
+        CommentSpacingRule(),
         FinalNewlineRule(),
         // disabled until it's clear how to reconcile difference in Intellij & Android Studio import layout
         // ImportOrderingRule(),

ktlint-ruleset-standard/src/test/kotlin/com/github/shyiko/ktlint/ruleset/standard/CommentSpacingRuleTest.kt

@@ -0,0 +1,63 @@
+package com.github.shyiko.ktlint.ruleset.standard
+
+import com.github.shyiko.ktlint.core.LintError
+import com.github.shyiko.ktlint.test.format
+import com.github.shyiko.ktlint.test.lint
+import org.assertj.core.api.Assertions.assertThat
+import org.testng.annotations.Test
+
+class CommentSpacingRuleTest {
+
+    @Test
+    fun testLintValidCommentSpacing() {
+        assertThat(CommentSpacingRule().lint(
+            """
+                // comment
+                var debugging = false // comment
+                var debugging = false // comment//word
+                    // comment
+            """.trimIndent()
+        )).isEmpty()
+    }
+
+    @Test
+    fun testLintInvalidCommentSpacing() {
+        assertThat(CommentSpacingRule().lint(
+            """
+                //comment
+                var debugging = false// comment
+                var debugging = false //comment
+                var debugging = false//comment
+                    //comment
+            """.trimIndent()
+        )).isEqualTo(listOf(
+            LintError(1, 1, "comment-spacing", "Missing space after double slash in end of line comment"),
+            LintError(2, 22, "comment-spacing", "Missing space before end of line comment"),
+            LintError(3, 23, "comment-spacing", "Missing space after double slash in end of line comment"),
+            LintError(4, 22, "comment-spacing", "Missing space before end of line comment"),
+            LintError(4, 22, "comment-spacing", "Missing space after double slash in end of line comment"),
+            LintError(5, 5, "comment-spacing", "Missing space after double slash in end of line comment")
+        ))
+    }
+
+    @Test
+    fun testFormatInvalidCommentSpacing() {
+        assertThat(CommentSpacingRule().format(
+            """
+                //comment
+                var debugging = false// comment
+                var debugging = false //comment
+                var debugging = false//comment
+                    //comment
+            """.trimIndent()
+        )).isEqualTo(
+            """
+                // comment
+                var debugging = false // comment
+                var debugging = false // comment
+                var debugging = false // comment
+                    // comment
+            """.trimIndent()
+        )
+    }
+}