fixes #764 (#1008)

xml/chapter3/section3/subsection3.xml

@@ -686,6 +686,131 @@ const put = operation_table("insert");
     </SPLITINLINE>
     should take as input a list of keys used to access the table.
     <LABEL NAME="ex:3_25"/>
+    <SOLUTION>
+      <SNIPPET>
+	<NAME>solution_3_25</NAME>
+	<EXAMPLE>solution_3_25_example</EXAMPLE>
+	<JAVASCRIPT>
+// contributed by GitHub user tttinkl
+
+function assoc(key, records, same_key) {
+  return is_null(records)
+    ? undefined
+    : same_key(key, head(head(records)))
+    ? head(records)
+    : assoc(key, tail(records), same_key);
+}
+
+
+function make_table(same_key) {
+  const local_table = list("*table");
+
+  const get_value = tail;
+
+  function is_table(t) {
+    return is_pair(t) &amp;&amp; head(t) === "*table";
+  }
+
+  function lookup(keys) {
+    function lookup_generic(keys, table) {
+      if (is_null(keys)) {
+        return table;
+      }
+      const key_1 = head(keys);
+      const key_rest = tail(keys);
+      const record = assoc(key_1, tail(table), same_key);
+      if (is_undefined(record)) {
+        return undefined;
+      }
+      if (is_null(key_rest)) {
+        return get_value(record);
+      } else if (is_table(get_value(record))) {
+        return lookup_generic(key_rest, get_value(record));
+      } else {
+        error('invalid key');
+      }
+    }
+    return lookup_generic(keys, local_table);
+  }
+
+
+  function insert(keys, value) {
+    function insert_generic(keys, value, table) {
+      const key_1 = head(keys);
+      const key_rest = tail(keys);
+      const record = assoc(key_1, tail(table), same_key);
+      if (is_undefined(record)) {
+        if (is_null(key_rest)) {
+          set_tail(
+            table,
+            pair(pair(key_1, value), tail(table)));
+        } else {
+          const new_subtable = list("*table");
+          set_tail(
+            table,
+            pair(pair(key_1, new_subtable), tail(table))
+          );
+          insert_generic(key_rest, value, new_subtable);
+        }
+      } else {
+        if (is_null(key_rest)) {
+          set_tail(record, value);
+        } else {
+          if (is_table(get_value(record))) {
+            insert_generic(key_rest, value, get_value(record));
+          } else {
+            const new_subtable = list("*table");
+            set_tail(record, new_subtable);
+            insert_generic(key_rest, value, new_subtable);
+          }
+        }
+      }
+    }
+    insert_generic(keys, value, local_table);
+  }
+
+  function dispatch(m) {
+    return m === "lookup"
+      ? lookup
+      : m === "insert"
+      ? insert
+      : m === "show"
+      ? () => {
+        display(local_table);
+        return local_table;
+      }
+      : error(m, "unknow operation -- table");
+  }
+  return dispatch;
+}
+
+const table = make_table(equal);
+
+const get = table('lookup');
+const put = table('insert');
+const show = table('show');
+	</JAVASCRIPT>
+      </SNIPPET>
+      <SNIPPET HIDE="yes">
+	<NAME>solution_3_25_example</NAME>
+	<JAVASCRIPT>
+put(list("a"), 1);
+put(list("b", "c"), 2);
+put(list("d", "e", "f"), 3);
+
+display(get(list("a")));
+display(get(list("b", "c")));
+display(get(list("d", "e", "f")));
+
+put(list("a", "b"), 1);
+display(get(list("a")));
+put(list("b", "c", "d"), 2);
+display(get(list("b", "c")));
+put(list("b"), 1);
+display(get(list("b")));
+	</JAVASCRIPT>
+      </SNIPPET>
+    </SOLUTION>
   </EXERCISE>
 
   <EXERCISE>