feat: Add selectHeadlines function

nix/default.nix

@@ -1,6 +1,7 @@
 with builtins;
 let
   excludeOrgSubtreesOnHeadlines = import ./excludeOrgSubtreesOnHeadlines.nix;
+  selectHeadlines = import ./selectHeadlines.nix;
 
   matchOrgTag = import ./matchOrgTag.nix;
   matchOrgHeadline = import ./matchOrgHeadline.nix;
@@ -12,6 +13,7 @@ in
 {
   # Newer concise APIs
   excludeHeadlines = excludeOrgSubtreesOnHeadlines;
+  inherit selectHeadlines;
   tag = matchOrgTag;
   headlineText = matchOrgHeadline;
   allP = predicates: x: builtins.all (p: p x) predicates;

nix/selectHeadlines.nix

@@ -0,0 +1,35 @@
+# Select headlines matching a tag/headline
+pred:
+with builtins;
+let
+  inherit (import ./utils.nix)
+    makeSubtreeEndRegexp
+    getHeadlineLevel
+    splitListWith
+    dropWhile
+    isHeadline;
+
+  makeEndPred =  headline:
+    s: isHeadline s
+       && match (makeSubtreeEndRegexp (getHeadlineLevel headline)) s != null;
+
+  go0 = lines:
+    if length lines == 0
+    then [ ]
+    else go1 (head lines) (splitListWith (makeEndPred (head lines)) (tail lines));
+
+  go1 = initial: cut:
+    [ initial ]
+    ++
+    cut.before
+    ++
+    (if cut.sep == null
+     then [ ]
+     else go ([ cut.sep ] ++ cut.after));
+
+  go = lines:
+    if length lines == 0
+    then [ ]
+    else go0 (dropWhile (s: !(isHeadline s && pred s)) lines);
+in
+go

test/test.nix

@@ -3,10 +3,11 @@ with builtins;
 let
   pkgs = import <nixpkgs> { };
   exclude = import ../nix/excludeOrgSubtreesOnHeadlines.nix;
+  select = import ../nix/selectHeadlines.nix;
   matchOrgTag = import ../nix/matchOrgTag.nix;
   matchOrgHeadline = import ../nix/matchOrgHeadline.nix;
   dropUntil = import ../nix/dropUntil.nix;
-  lines = filter isString (split "\n" (readFile ./test.org));
+  lines = filter (s: isString s && s != "") (split "\n" (readFile ./test.org));
 in
 pkgs.lib.runTests {
   testTagPredicate = {
@@ -28,4 +29,14 @@ pkgs.lib.runTests {
     expr = pkgs.lib.last (exclude (matchOrgTag "optional") lines);
     expected = "It was a good day!";
   };
+
+  testSelect1 = {
+    expr = pkgs.lib.last (select (matchOrgTag "irregular") lines);
+    expected = "Why did he come to the office in the first place?";
+  };
+
+  testSelect2 = {
+    expr = pkgs.lib.last (select (matchOrgTag "optional") lines);
+    expected = "/Zaijian/ means goodbye in Mandarin Chinese.";
+  };
 }