Add string::positionToLocation() and positionToLocation(). Handle up/down keys in TextField.

include/string.hpp

@@ -47,6 +47,16 @@ bool startsWith(const std::string& str, const std::string& prefix);
 /** Returns whether a string ends with the given substring. */
 bool endsWith(const std::string& str, const std::string& suffix);
 
+struct Location {
+	/** Line number, 0-indexed */
+	size_t line;
+	/** UTF-8 codepoint index, 0-indexed */
+	size_t column;
+};
+/** Given a byte position of s, returns the 2D UTF-8 location of the cursor. */
+Location positionToLocation(const std::string& s, size_t pos);
+size_t locationToPosition(const std::string& s, Location location);
+
 /** Converts a byte array to a Base64-encoded string.
 https://en.wikipedia.org/wiki/Base64
 */

src/string.cpp

@@ -155,6 +155,52 @@ bool endsWith(const std::string& str, const std::string& suffix) {
 }
 
 
+Location positionToLocation(const std::string& s, size_t pos) {
+	if (pos > s.size())
+		pos = s.size();
+
+	// Count number of newlines until pos
+	size_t line = 0;
+	size_t linePos = 0;
+	for (size_t i = 0; i < pos; i++) {
+		if (s[i] == '\n') {
+			line++;
+			linePos = i + 1;
+		}
+	}
+
+	// Count number of codepoints from line until pos
+	size_t column = 0;
+	for (size_t i = linePos; i < pos;) {
+		i = UTF8NextCodepoint(s, i);
+		column++;
+	}
+
+	return {line, column};
+}
+
+
+size_t locationToPosition(const std::string& s, Location location) {
+	size_t pos = 0;
+
+	// Advance `location.line` newlines
+	for (size_t lines = 0; lines < location.line && pos < s.size(); pos++) {
+		if (s[pos] == '\n')
+			lines++;
+	}
+
+	// Advance `location.column` codepoints
+	for (size_t columns = 0; columns < location.column && pos < s.size(); columns++) {
+		// Stop if column is beyond newline
+		if (s[pos] == '\n')
+			break;
+		pos = UTF8NextCodepoint(s, pos);
+	}
+
+	return pos;
+}
+
+
 std::string toBase64(const uint8_t* data, size_t dataLen) {
 	static const char alphabet[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
 

src/ui/TextField.cpp

@@ -198,10 +198,21 @@ void TextField::onSelectKey(const SelectKeyEvent& e) {
 		}
 		// Up (placeholder)
 		if (e.isKeyCommand(GLFW_KEY_UP)) {
+			string::Location location = string::positionToLocation(text, cursor);
+			if (location.line == 0)
+				location.column = 0;
+			else
+				location.line--;
+			cursor = locationToPosition(text, location);
+			selection = cursor;
 			e.consume(this);
 		}
 		// Down (placeholder)
 		if (e.isKeyCommand(GLFW_KEY_DOWN)) {
+			string::Location location = string::positionToLocation(text, cursor);
+			location.line++;
+			cursor = locationToPosition(text, location);
+			selection = cursor;
 			e.consume(this);
 		}
 		// Home