Introduce ThingPickerParser.

To ensure that we are extensible from the get-go, this commit introduces
the parser aspect of the ThingPicker framework with parsers for the two
non-trivial picker implementations.

Nothing is wired up yet.

src/main/java/com/garbagemule/MobArena/things/ParserUtil.java

@@ -0,0 +1,95 @@
+package com.garbagemule.MobArena.things;
+
+import java.util.ArrayList;
+import java.util.List;
+
+class ParserUtil {
+
+    static String extractBetween(String s, char left, char right) {
+        int start = s.indexOf(left);
+        if (start < 0) {
+            throw new IllegalArgumentException("Missing start symbol " + left);
+        }
+
+        int end = s.lastIndexOf(right);
+        if (end < 0) {
+            throw new IllegalArgumentException("Missing end symbol " + right);
+        }
+
+        return s.substring(start + 1, end).trim();
+    }
+
+    static List<String> split(String s) {
+        List<String> result = new ArrayList<>();
+        int start = 0;
+        int parens = 0;
+        int brackets = 0;
+        int curlies = 0;
+        int angles = 0;
+        for (int i = 0; i < s.length(); i++) {
+            char c = s.charAt(i);
+            if (c == ',') {
+                if (parens == 0 && brackets == 0 && curlies == 0 && angles == 0) {
+                    String part = s.substring(start, i).trim();
+                    if (!part.isEmpty()) {
+                        result.add(part);
+                    }
+                    start = i + 1;
+                }
+            } else if (c == '(') {
+                parens++;
+            } else if (c == ')') {
+                parens--;
+                if (parens < 0) {
+                    throw new IllegalArgumentException("Unmatched right parenthesis )");
+                }
+            } else if (c == '[') {
+                brackets++;
+            } else if (c == ']') {
+                brackets--;
+                if (brackets < 0) {
+                    throw new IllegalArgumentException("Unmatched right square bracket ]");
+                }
+            } else if (c == '{') {
+                curlies++;
+            } else if (c == '}') {
+                curlies--;
+                if (curlies < 0) {
+                    throw new IllegalArgumentException("Unmatched right curly brace }");
+                }
+            } else if (c == '<') {
+                angles++;
+            } else if (c == '>') {
+                angles--;
+                if (angles < 0) {
+                    throw new IllegalArgumentException("Unmatched right angle bracket >");
+                }
+            }
+        }
+        if (parens > 0) {
+            throw new IllegalArgumentException("Unmatched left parenthesis (");
+        }
+        if (brackets > 0) {
+            throw new IllegalArgumentException("Unmatched left square bracket [");
+        }
+        if (curlies > 0) {
+            throw new IllegalArgumentException("Unmatched left curly brace {");
+        }
+        if (angles > 0) {
+            throw new IllegalArgumentException("Unmatched left angle bracket <");
+        }
+        if (start == 0) {
+            String part = s.trim();
+            if (!part.isEmpty()) {
+                result.add(part);
+            }
+        } else {
+            String part = s.substring(start).trim();
+            if (!part.isEmpty()) {
+                result.add(part);
+            }
+        }
+        return result;
+    }
+
+}

src/main/java/com/garbagemule/MobArena/things/RandomThingPickerParser.java

@@ -0,0 +1,43 @@
+package com.garbagemule.MobArena.things;
+
+import java.util.List;
+import java.util.Random;
+import java.util.stream.Collectors;
+
+public class RandomThingPickerParser implements ThingPickerParser {
+
+    private final ThingPickerParser parser;
+    private final Random random;
+
+    public RandomThingPickerParser(
+        ThingPickerParser parser,
+        Random random
+    ) {
+        this.parser = parser;
+        this.random = random;
+    }
+
+    @Override
+    public ThingPicker parse(String s) {
+        if (!(s.startsWith("random(") && s.endsWith(")"))) {
+            return null;
+        }
+
+        String inner = ParserUtil.extractBetween(s, '(', ')');
+        List<ThingPicker> pickers = ParserUtil.split(inner)
+            .stream()
+            .map(String::trim)
+            .map(parser::parse)
+            .collect(Collectors.toList());
+
+        if (pickers.isEmpty()) {
+            throw new IllegalArgumentException("Nothing to pick from: " + s);
+        }
+        if (pickers.size() == 1) {
+            return pickers.get(0);
+        }
+
+        return new RandomThingPicker(pickers, random);
+    }
+
+}

src/main/java/com/garbagemule/MobArena/things/ThingGroupPickerParser.java

@@ -0,0 +1,37 @@
+package com.garbagemule.MobArena.things;
+
+import java.util.List;
+import java.util.stream.Collectors;
+
+public class ThingGroupPickerParser implements ThingPickerParser {
+
+    private final ThingPickerParser parser;
+
+    public ThingGroupPickerParser(ThingPickerParser parser) {
+        this.parser = parser;
+    }
+
+    @Override
+    public ThingPicker parse(String s) {
+        if (!(s.startsWith("all(") && s.endsWith(")"))) {
+            return null;
+        }
+
+        String inner = ParserUtil.extractBetween(s, '(', ')');
+        List<ThingPicker> pickers = ParserUtil.split(inner)
+            .stream()
+            .map(String::trim)
+            .map(parser::parse)
+            .collect(Collectors.toList());
+
+        if (pickers.isEmpty()) {
+            throw new IllegalArgumentException("Nothing to group: " + s);
+        }
+        if (pickers.size() == 1) {
+            return pickers.get(0);
+        }
+
+        return new ThingGroupPicker(pickers);
+    }
+
+}

src/main/java/com/garbagemule/MobArena/things/ThingPickerParser.java

@@ -0,0 +1,14 @@
+package com.garbagemule.MobArena.things;
+
+public interface ThingPickerParser {
+
+    /**
+     * Parse the given string, returning a {@link ThingPicker} instance on
+     * success, otherwise null.
+     *
+     * @param s a string to parse
+     * @return an instance of {@link ThingPicker}, or null
+     */
+    ThingPicker parse(String s);
+
+}