Indentation string calculations

Author: mattmassicotte

Sources/TextFormation/Indentation.swift

@@ -30,3 +30,27 @@ public enum IndentationError: Error {
 
 extension IndentationError: Hashable {}
 extension IndentationError: Sendable {}
+
+extension Indentation {
+	/// Apply an indentation operation to a whitespace string.
+	public func apply(to string: String, indentationUnit: String, width: Int) -> String {
+		// here, we have to determine how many units of indentation currently exist
+		let spaceOnlyReference = string.replacingOccurrences(of: "\t", with: String(repeating: " ", count: width))
+		let spaceCount = spaceOnlyReference.utf8.count
+		let referenceCount = spaceCount / width
+		let remainder = spaceCount % width
+
+		switch self {
+		case .equal:
+			return String(repeating: indentationUnit, count: referenceCount) + String(repeating: " ", count: remainder)
+		case .relativeIncrease:
+			return String(repeating: indentationUnit, count: referenceCount + 1) + String(repeating: " ", count: remainder)
+		case .relativeDecrease:
+			if referenceCount == 0 {
+				return string
+			}
+			
+			return String(repeating: indentationUnit, count: referenceCount - 1) + String(repeating: " ", count: remainder)
+		}
+	}
+}

Tests/TextFormationTests/IndentationTests.swift

@@ -0,0 +1,70 @@
+import Foundation
+import Testing
+
+import TextFormation
+
+struct IndentationTests {
+	@Test func equalEmpty() throws {
+		let equal = Indentation.equal(NSRange.zero)
+
+		#expect(equal.apply(to: "", indentationUnit: "\t", width: 4) == "")
+		#expect(equal.apply(to: "", indentationUnit: "  ", width: 4) == "")
+		#expect(equal.apply(to: "", indentationUnit: "    ", width: 4) == "")
+	}
+
+	@Test func increaseEmpty() throws {
+		let equal = Indentation.relativeIncrease(NSRange.zero)
+
+		#expect(equal.apply(to: "", indentationUnit: "\t", width: 4) == "\t")
+		#expect(equal.apply(to: "", indentationUnit: "  ", width: 4) == "  ")
+		#expect(equal.apply(to: "", indentationUnit: "    ", width: 4) == "    ")
+	}
+
+	@Test func decreaseEmpty() throws {
+		let equal = Indentation.relativeDecrease(NSRange.zero)
+
+		#expect(equal.apply(to: "", indentationUnit: "\t", width: 4) == "")
+		#expect(equal.apply(to: "", indentationUnit: "  ", width: 4) == "")
+		#expect(equal.apply(to: "", indentationUnit: "    ", width: 4) == "")
+	}
+
+	@Test func equalTab() throws {
+		let equal = Indentation.equal(NSRange.zero)
+
+		#expect(equal.apply(to: "\t", indentationUnit: "\t", width: 4) == "\t")
+		#expect(equal.apply(to: "\t", indentationUnit: "  ", width: 4) == "  ")
+		#expect(equal.apply(to: "\t", indentationUnit: "    ", width: 4) == "    ")
+	}
+
+	@Test func equalFourSpace() throws {
+		let equal = Indentation.equal(NSRange.zero)
+
+		#expect(equal.apply(to: "    ", indentationUnit: "\t", width: 4) == "\t")
+		#expect(equal.apply(to: "    ", indentationUnit: "  ", width: 4) == "  ")
+		#expect(equal.apply(to: "    ", indentationUnit: "    ", width: 4) == "    ")
+	}
+
+	@Test func decreaseFourSpace() throws {
+		let equal = Indentation.relativeDecrease(NSRange.zero)
+
+		#expect(equal.apply(to: "    ", indentationUnit: "\t", width: 4) == "")
+		#expect(equal.apply(to: "    ", indentationUnit: "  ", width: 2) == "  ")
+		#expect(equal.apply(to: "    ", indentationUnit: "    ", width: 4) == "")
+	}
+
+	@Test func increaseFourSpace() throws {
+		let equal = Indentation.relativeIncrease(NSRange.zero)
+
+		#expect(equal.apply(to: "    ", indentationUnit: "\t", width: 4) == "\t\t")
+		#expect(equal.apply(to: "    ", indentationUnit: "  ", width: 2) == "      ")
+		#expect(equal.apply(to: "    ", indentationUnit: "    ", width: 4) == "        ")
+	}
+
+	@Test func increasePreservingAlignment() throws {
+		let equal = Indentation.relativeIncrease(NSRange.zero)
+
+		#expect(equal.apply(to: "\t ", indentationUnit: "\t", width: 4) == "\t\t ")
+		#expect(equal.apply(to: "   ", indentationUnit: "  ", width: 2) == "     ")
+		#expect(equal.apply(to: "     ", indentationUnit: "    ", width: 4) == "         ")
+	}
+}