Unify handling of command-less input in ProBKernel

There is now a single preprocessInput method that is responsible for
adding a command prefix to any input that doesn't already have one.
This simplifies the implementations of (eval|inspect|complete)Internal
somewhat, because they can now assume that the input (after
preprocessing) always starts with a command name.

src/main/java/de/prob2/jupyter/ProBKernel.java

@@ -322,27 +322,47 @@ public final class ProBKernel extends BaseKernel {
 		return MACHINE_CODE_PATTERN.matcher(code).matches();
 	}
 	
-	private @Nullable DisplayData evalInternal(final @NotNull PositionedString code) {
+	/**
+	 * Preprocess the given input by ensuring that it starts with a command name.
+	 * If a command name is already present,
+	 * the input is returned unchanged.
+	 * Otherwise,
+	 * an appropriate command is added based on the type of input.
+	 * 
+	 * @param code the input code to preprocess
+	 * @return the input with a command name prefixed if necessary
+	 */
+	private static @NotNull PositionedString preprocessInput(final @NotNull PositionedString code) {
 		final Matcher commandMatcher = COMMAND_PATTERN.matcher(code.getValue());
-		final PositionedString name;
-		final PositionedString argString;
+		final String prefix;
 		if (commandMatcher.matches()) {
-			// The input is a command, execute it directly.
-			name = code.substring(commandMatcher.start(1), commandMatcher.end(1));
-			if (commandMatcher.group(2) == null) {
-				argString = code.substring(code.getValue().length());
+			// The input already includes a command, so no prefix needs to be added.
+			prefix = "";
+		} else if (isMachineCode(code.getValue())) {
+			// The input appears to be a machine, add a command to load it.
+			prefix = "::load\n";
 		} else {
-				argString = code.substring(commandMatcher.start(2), commandMatcher.end(2));
+			// By default, assume that the input is an expression that should be evaluated.
+			prefix = ":eval ";
 		}
-		} else if (isMachineCode(code.getValue())) {
-			// The input appears to be a machine, load it.
-			// The leading newline here is important. ::load expects the first input line to contain preference assignments; the actual machine code has to start on the second line.
-			name = new PositionedString("::load", code.getStartPosition() - 7);
-			argString = new PositionedString("\n" + code.getValue(), code.getStartPosition() - 1);
+		// Add the prefix and adjust the start position.
+		// This means that the characters from the prefix (if any) will have negative positions,
+		// but the characters from the real source code will have the same positions as before.
+		return new PositionedString(prefix + code.getValue(), code.getStartPosition() - prefix.length());
+	}
+	
+	private @Nullable DisplayData evalInternal(final @NotNull PositionedString code) {
+		final PositionedString preprocessedCode = preprocessInput(code);
+		final Matcher commandMatcher = COMMAND_PATTERN.matcher(preprocessedCode.getValue());
+		if (!commandMatcher.matches()) {
+			throw new AssertionError("Preprocessed input does not include a command - this should not happen");
+		}
+		final PositionedString name = preprocessedCode.substring(commandMatcher.start(1), commandMatcher.end(1));
+		final PositionedString argString;
+		if (commandMatcher.group(2) == null) {
+			argString = preprocessedCode.substring(preprocessedCode.getValue().length());
 		} else {
-			// By default, assume that the input is an expression and evaluate it.
-			name = new PositionedString(":eval", code.getStartPosition() - 6);
-			argString = code;
+			argString = preprocessedCode.substring(commandMatcher.start(2), commandMatcher.end(2));
 		}
 		return this.executeCommand(name, argString);
 	}
@@ -378,16 +398,18 @@ public final class ProBKernel extends BaseKernel {
 	}
 	
 	private @Nullable DisplayData inspectInternal(final @NotNull PositionedString code, final int at) {
-		final Matcher commandMatcher = COMMAND_PATTERN.matcher(code.getValue());
-		if (commandMatcher.matches()) {
-			// The code is a valid command.
+		final PositionedString preprocessedCode = preprocessInput(code);
+		final Matcher commandMatcher = COMMAND_PATTERN.matcher(preprocessedCode.getValue());
+		if (!commandMatcher.matches()) {
+			throw new AssertionError("Preprocessed input does not include a command - this should not happen");
+		}
 		final String name = commandMatcher.group(1);
 		if (this.getCommands().containsKey(name)) {
 			final Command command = this.getCommands().get(name);
-				if (at <= commandMatcher.end(1)) {
+			if (at <= preprocessedCode.getStartPosition() + commandMatcher.end(1)) {
 				// The cursor is somewhere in the command name, show help text for the command.
 				return command.renderHelp();
-				} else if (at < commandMatcher.start(2)) {
+			} else if (at < preprocessedCode.getStartPosition() + commandMatcher.start(2)) {
 				// The cursor is in the whitespace between the command name and arguments, don't show anything.
 				return null;
 			} else {
@@ -395,9 +417,9 @@ public final class ProBKernel extends BaseKernel {
 				assert name != null;
 				final PositionedString argString;
 				if (commandMatcher.group(2) == null) {
-						argString = code.substring(code.getValue().length());
+					argString = preprocessedCode.substring(preprocessedCode.getValue().length());
 				} else {
-						argString = code.substring(commandMatcher.start(2), commandMatcher.end(2));
+					argString = preprocessedCode.substring(commandMatcher.start(2), commandMatcher.end(2));
 				}
 				return inspectCommandArguments(command, argString, at);
 			}
@@ -405,10 +427,6 @@ public final class ProBKernel extends BaseKernel {
 			// Invalid command, can't inspect.
 			return null;
 		}
-		} else {
-			// The code is not a valid command, ask :eval to inspect.
-			return inspectCommandArguments(this.getCommands().get(":eval"), code, at);
-		}
 	}
 	
 	@Override
@@ -444,18 +462,20 @@ public final class ProBKernel extends BaseKernel {
 	}
 	
 	private @Nullable ReplacementOptions completeInternal(final @NotNull PositionedString code, final int at) {
-		final Matcher commandMatcher = COMMAND_PATTERN.matcher(code.getValue());
-		if (commandMatcher.matches()) {
-			// The code is a valid command.
-			if (at <= commandMatcher.end(1)) {
+		final PositionedString preprocessedCode = preprocessInput(code);
+		final Matcher commandMatcher = COMMAND_PATTERN.matcher(preprocessedCode.getValue());
+		if (!commandMatcher.matches()) {
+			throw new AssertionError("Preprocessed input does not include a command - this should not happen");
+		}
+		if (at <= preprocessedCode.getStartPosition() + commandMatcher.end(1)) {
 			// The cursor is somewhere in the command name, provide command completions.
-				final String prefix = code.substring(commandMatcher.start(1), at).getValue();
+			final String prefix = preprocessedCode.substring(commandMatcher.start(1), at).getValue();
 			return new ReplacementOptions(
 				this.getCommands().keySet().stream().filter(s -> s.startsWith(prefix)).sorted().collect(Collectors.toList()),
 				commandMatcher.start(1), 
 				commandMatcher.end(1)
 			);
-			} else if (at < commandMatcher.start(2)) {
+		} else if (at < preprocessedCode.getStartPosition() + commandMatcher.start(2)) {
 			// The cursor is in the whitespace between the command name and arguments, don't show anything.
 			return null;
 		} else {
@@ -464,9 +484,9 @@ public final class ProBKernel extends BaseKernel {
 			assert name != null;
 			final PositionedString argString;
 			if (commandMatcher.group(2) == null) {
-					argString = code.substring(code.getValue().length());
+				argString = preprocessedCode.substring(preprocessedCode.getValue().length());
 			} else {
-					argString = code.substring(commandMatcher.start(2), commandMatcher.end(2));
+				argString = preprocessedCode.substring(commandMatcher.start(2), commandMatcher.end(2));
 			}
 			if (this.getCommands().containsKey(name)) {
 				return completeCommandArguments(this.getCommands().get(name), argString, at);
@@ -475,10 +495,6 @@ public final class ProBKernel extends BaseKernel {
 				return null;
 			}
 		}
-		} else {
-			// The code is not a valid command, ask :eval for completions.
-			return completeCommandArguments(this.getCommands().get(":eval"), code, at);
-		}
 	}
 	
 	@Override