Implement function

src/main/java/org/itsallcode/jlua/LowLevelLua.java

@@ -68,11 +68,20 @@ LuaStack stack() {
     }
 
     LuaTable table(final int idx) {
+        assertType(idx, LuaType.TABLE);
+        return new LuaTable(state, stack, arena, idx);
+    }
+
+    public LuaFunction function(final int idx) {
+        assertType(idx, LuaType.FUNCTION);
+        return new LuaFunction(state, stack, arena, idx);
+    }
+
+    private void assertType(final int idx, final LuaType expectedType) {
         final LuaType type = stack().getType(idx);
-        if (type != LuaType.TABLE) {
-            throw new LuaException("Expected table on the stack but got " + type);
+        if (type != expectedType) {
+            throw new LuaException("Expected " + expectedType + " at " + idx + " but got " + type);
         }
-        return new LuaTable(state, stack, arena, idx);
     }
 
     boolean isYieldable() {

src/main/java/org/itsallcode/jlua/LuaFunction.java

@@ -0,0 +1,44 @@
+package org.itsallcode.jlua;
+
+import java.lang.foreign.Arena;
+import java.lang.foreign.MemorySegment;
+
+import org.itsallcode.jlua.ffi.Lua;
+
+public class LuaFunction {
+
+    private final MemorySegment state;
+    private final LuaStack stack;
+    private final Arena arena;
+    private final int idx;
+
+    LuaFunction(final MemorySegment state, final LuaStack stack, final Arena arena, final int idx) {
+        this.state = state;
+        this.stack = stack;
+        this.arena = arena;
+        this.idx = idx;
+    }
+
+    public void addArgInteger(final int value) {
+        stack.pushInteger(value);
+    }
+
+    public void call(final int nargs, final int nresults) {
+        call(nargs, nresults, 0, 0, Lua.NULL());
+    }
+
+    private void call(final int nargs, final int nresults, final int errfunc, final long ctx, final MemorySegment k) {
+        final int status = Lua.lua_pcallk(state, nargs, nresults, errfunc, ctx, k);
+        if (status != Lua.LUA_OK()) {
+            final String errorMessage = stack.toString(-1);
+            stack.pop(1);
+            throw new FunctionCallException("lua_pcallk", status, errorMessage);
+        }
+    }
+
+    public long getIntegerResult() {
+        final long value = stack.toInteger(-1);
+        stack.pop(1);
+        return value;
+    }
+}

src/main/java/org/itsallcode/jlua/LuaInterpreter.java

@@ -51,6 +51,11 @@ public LuaTable getGlobalTable(final String name) {
         return lua.table(-1);
     }
 
+    public LuaFunction getGlobalFunction(final String name) {
+        lua.getGlobal(name);
+        return lua.function(-1);
+    }
+
     public void setGlobalString(final String name, final String value) {
         lua.stack().pushString(value);
         lua.setGlobal(name);

src/test/java/org/itsallcode/jlua/LuaInterpreterTest.java

@@ -91,6 +91,25 @@ void setGetGlobalBoolean(final boolean value) {
         assertThat(globalBoolean, equalTo(value));
     }
 
+    @Test
+    void getCallGlobalFunction() {
+        lua.exec("function increment(x) return x+1 end");
+        final LuaFunction function = lua.getGlobalFunction("increment");
+        function.addArgInteger(42);
+        function.call(1, 1);
+        assertThat(function.getIntegerResult(), equalTo(43L));
+    }
+
+    @Test
+    void getCallGlobalFunctionFails() {
+        lua.exec("function increment(x) error('failure') end");
+        final LuaFunction function = lua.getGlobalFunction("increment");
+        function.addArgInteger(42);
+        final LuaException exception = assertThrows(LuaException.class, () -> function.call(1, 1));
+        assertThat(exception.getMessage(), equalTo(
+                "Function 'lua_pcallk' failed with error 2: [string \"function increment(x) error('failure') end\"]:1: failure"));
+    }
+
     @Test
     void getTableStringValue() {
         lua.exec("result = { key = 'value' }");
@@ -139,7 +158,7 @@ void getTableValueWrongType() {
     void getTableStringFailsWrongType() {
         lua.exec("result = 'not a table'");
         final LuaException exception = assertThrows(LuaException.class, () -> lua.getGlobalTable("result"));
-        assertThat(exception.getMessage(), equalTo("Expected table on the stack but got STRING"));
+        assertThat(exception.getMessage(), equalTo("Expected TABLE at -1 but got STRING"));
     }