From 612b479c3a861b1b6a0e03b98bba9aa1bc091247 Mon Sep 17 00:00:00 2001
From: Howard Abrams <howard.abrams@workday.com>
Date: Wed, 14 Jun 2023 17:36:30 -0700
Subject: [PATCH] Bring in code from a web essay for parsing data structures

---
 elisp/query-tree.el | 134 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 134 insertions(+)
 create mode 100644 elisp/query-tree.el

diff --git a/elisp/query-tree.el b/elisp/query-tree.el
new file mode 100644
index 0000000..9c4f572
--- /dev/null
+++ b/elisp/query-tree.el
@@ -0,0 +1,134 @@
+;;; query-tree.el --- query interface to data structures. -*- lexical-binding: t; -*-
+;;
+;; Copyright © 2023 Howard X. Abrams
+;;
+;; Author: Howard X. Abrams <http://gitlab.com/howardabrams>
+;; Maintainer: Howard X. Abrams <howard.abrams@gmail.com>
+;; Created: April 23, 2023
+;;
+;; This file is not part of GNU Emacs.
+;;
+;;; Commentary:
+;;
+;; A Lisp-focused approach to extracting values from a complex data
+;; structure, similar to how jq works with JSON objects.
+;;
+;; For instance, given a _parsed_ JSON object (a collection of
+;; hash-tables and vectors), like (stored in variable data):
+;;
+;;   { "a": 1,
+;;     "b": {
+;;        "x": 10,
+;;        "y": [100, 101, 102, 103],
+;;        "z": 30
+;;     },
+;;     "c": 3
+;;   }
+;;
+;; We should be able to call:  (query-tree data 'a 'b 2)
+;; Should return 102.
+;;
+;; *NB:* Do not edit this file. Instead, edit the original literate file at:
+;;            ~/website/Technical/Emacs/query-data-structures.org
+;;       And tangle the file to recreate this one.
+;;
+;;; Code:
+
+(defun query-tree (data-structure &rest query)
+  "Return value from DATA-STRUCTURE based on elements of QUERY."
+  (query-tree-parse query data-structure))
+
+(defun query-tree-parse (query tree)
+  "Return value of QUERY from TREE data structure."
+  (if (null query)
+      tree
+    (query-tree-parse (cdr query)
+                      (query-tree--parse-node (car query) tree))))
+
+(defun query-tree--parse-node (node tree)
+  "Return results of TREE data structure of symbol, NODE.
+
+If TREE is a hash-table, then NODE would be the key, either a
+string or a symbol. Return value.
+
+If TREE is a sequence and NODE is number, Return element indexed
+by NODE.
+
+If TREE is a Property List, return value of NODE as a key. This
+should work with association lists as well."
+  (cond
+   ((null tree)                        nil)
+   ((hash-table-p tree)                 (or (gethash node tree)
+                                            (gethash (symbol-name node) tree)))
+   ((and (numberp node)  (seqp tree))       (seq-elt tree node))
+   ((and (eq node :first) (seqp tree))      (car node))
+   ((and (eq node :last) (seqp tree))       (last node))
+
+   ((assoc node tree)                       (alist-get node tree))
+   ((plistp tree)                           (plist-get tree node))
+   (t                                  (error "query-tree: Looking for '%s' in %s" node tree))))
+
+(ert-deftest query-tree-walk-tree-test ()
+  ;; PLIST:
+  (let ((data '(:a 1 :b 2 :c 3)))
+    (should (eq (query-tree--parse-node :b data) 2)))
+  ;; ALIST:
+  (let ((data '((pine . cones) (oak . acorns) (maple . seeds))))
+    (should (eq (query-tree--parse-node 'oak data) 'acorns)))
+  ;; LIST, aka a sequence
+  (let ((data '(a b c d e f)))
+    (should (eq (query-tree--parse-node 3 data) 'd)))
+  ;; VECTOR:
+  (let ((data [a b c d e]))
+    (should (eq (query-tree--parse-node 2 data) 'c)))
+  ;; HASH TABLE with strings:
+  (let ((h (make-hash-table :test 'equal)))
+    (puthash "a" 1 h)
+    (puthash "b" 2 h)
+    (puthash "c" 3 h)
+    (should (eq (query-tree--parse-node "b" h) 2))
+    (should (eq (query-tree--parse-node 'b h) 2)))
+  ;; HASH TABLE with symbols:
+  (let ((h (make-hash-table)))
+    (puthash 'a 1 h)
+    (puthash 'b 2 h)
+    (puthash 'c 3 h)
+    (should (eq (query-tree--parse-node 'b h) 2))))
+
+(ert-deftest query-tree-parse-tree-test ()
+  (should (null (query-tree-parse nil nil)))
+  (let ((data :final))
+    (should (eq (query-tree-parse nil data) :final)))
+  (let ((data [ a b c d e f ]))
+    (should (eq (query-tree-parse '(3) data) 'd)))
+  (let ((data (json-parse-string
+               "{ \"a\": 1,
+                  \"b\": {
+                     \"x\": 10,
+                     \"y\": [100, 101, 102, 103],
+                     \"z\": 30
+                  },
+                  \"c\": 3
+                }")))
+    (should (hash-table-p data))
+    (should (= (query-tree-parse '("a") data) 1))
+    (should (= (query-tree-parse '("b" "x") data) 10))
+    (should (= (query-tree-parse '("b" "y" 2) data) 102))
+    (should (= (query-tree-parse '(a) data) 1))
+    (should (= (query-tree-parse '(b x) data) 10))
+    (should (= (query-tree-parse '(b y 2) data) 102))))
+
+(ert-deftest query-tree-test ()
+  (let ((data (json-parse-string
+               "{ \"a\": 1,
+                  \"b\": {
+                     \"x\": 10,
+                     \"y\": [100, 101, 102, 103],
+                     \"z\": 30
+                  },
+                  \"c\": 3
+                }")))
+    (should (= (query-tree data 'b 'y 2) 102))))
+
+(provide 'query-tree)
+;;; query-tree.el ends here