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+;;; coverage-core.el --- Coverage reporting engine and backend registry -*- lexical-binding: t; coding: utf-8; -*-
+;; author: Craig Jennings <c@cjennings.net>
+
+;;; Commentary:
+;; Language-agnostic core for diff-aware coverage reporting.
+;;
+;; Reads an LCOV file, shells to git diff at a selectable scope,
+;; intersects the results, and displays a report buffer.  Languages
+;; plug in via the backend registry (see `cj/coverage-backends').
+;;
+;; See docs/design/coverage.org for the design rationale.
+
+;;; Code:
+
+(defun cj/--coverage-parse-lcov (file)
+  "Parse FILE as LCOV and return a hash table of covered lines.
+Keys are source-file paths (strings).  Values are hash tables whose
+keys are line numbers (integers) that had a hit count greater than
+zero.  Only the SF, DA, and end_of_record fields are read; other
+LCOV fields are ignored.  Malformed DA lines are skipped silently.
+Signals `user-error' if FILE does not exist."
+  (unless (file-exists-p file)
+	(user-error "LCOV file not found: %s" file))
+  (let ((result (make-hash-table :test 'equal))
+		(current-file nil)
+		(current-lines nil))
+	(with-temp-buffer
+	  (insert-file-contents file)
+	  (goto-char (point-min))
+	  (while (not (eobp))
+		(let ((line (buffer-substring-no-properties
+					 (line-beginning-position) (line-end-position))))
+		  (cond
+		   ((string-prefix-p "SF:" line)
+			(setq current-file (substring line 3))
+			(setq current-lines (make-hash-table :test 'eql)))
+		   ((string-prefix-p "DA:" line)
+			(when current-lines
+			  (let* ((rest (substring line 3))
+					 (parts (split-string rest ","))
+					 (line-str (car parts))
+					 (hits-str (cadr parts))
+					 (line-num (and line-str (string-match-p "\\`[0-9]+\\'" line-str)
+									(string-to-number line-str)))
+					 (hits (and hits-str (string-match-p "\\`[0-9]+\\'" hits-str)
+								(string-to-number hits-str))))
+				(when (and line-num hits (> hits 0))
+				  (puthash line-num t current-lines)))))
+		   ((string= line "end_of_record")
+			(when current-file
+			  (puthash current-file current-lines result))
+			(setq current-file nil
+				  current-lines nil))))
+		(forward-line 1)))
+	result))
+
+(provide 'coverage-core)
+;;; coverage-core.el ends here