From 68cb9d0fbfad43c647a3da22834c79cd55f581a7 Mon Sep 17 00:00:00 2001
From: Oldes <oldes.huhuman@gmail.com>
Date: Fri, 8 Jul 2022 14:15:04 +0200
Subject: [PATCH] FEAT: implemented `indexz?`, `at` and `atz` actions on file
 port resolves: https://github.com/Oldes/Rebol-issues/issues/2506

---
 src/core/p-file.c            | 19 +++++++++++++++--
 src/tests/units/port-test.r3 | 40 ++++++++++++++++++++++++++++++++++++
 2 files changed, 57 insertions(+), 2 deletions(-)

diff --git a/src/core/p-file.c b/src/core/p-file.c
index 3b6a31622e..7ff193aac6 100644
--- a/src/core/p-file.c
+++ b/src/core/p-file.c
@@ -636,6 +636,9 @@ REBINT Mode_Syms[] = {
 	case A_INDEXQ:
 		SET_INTEGER(D_RET, file->file.index + 1);
 		break;
+	case A_INDEXZQ:
+		SET_INTEGER(D_RET, file->file.index);
+		break;
 
 	case A_LENGTHQ:
 		SET_INTEGER(D_RET, file->file.size - file->file.index); // !clip at zero
@@ -654,13 +657,20 @@ REBINT Mode_Syms[] = {
 		goto seeked;
 
 	case A_BACK:
-		if (file->file.index > 0) file->file.index--;
+		file->file.index--;
 		goto seeked;
 
 	case A_SKIP:
 		file->file.index += Get_Num_Arg(D_ARG(2));
 		goto seeked;
 
+	case A_AT:
+		file->file.index = Get_Num_Arg(D_ARG(2)) - 1;
+		goto seeked;
+	case A_ATZ:
+		file->file.index = Get_Num_Arg(D_ARG(2));
+		goto seeked;
+
     case A_HEADQ:
 		DECIDE(file->file.index == 0);
 
@@ -680,7 +690,6 @@ REBINT Mode_Syms[] = {
 		break;
 
 	/* Not yet implemented:
-		A_AT,					// 38
 		A_PICK,					// 41
 		A_PATH,					// 42
 		A_PATH_SET,				// 43
@@ -701,6 +710,12 @@ REBINT Mode_Syms[] = {
 	return R_RET;
 
 seeked:
+	// fit index in available range...
+	if (file->file.index < 0)
+		file->file.index = 0;
+	else if (file->file.index > file->file.size)
+		file->file.index = file->file.size;
+
 	SET_FLAG(file->modes, RFM_RESEEK);
 	return R_ARG1;
 
diff --git a/src/tests/units/port-test.r3 b/src/tests/units/port-test.r3
index f0184f4bcf..5c616f5822 100644
--- a/src/tests/units/port-test.r3
+++ b/src/tests/units/port-test.r3
@@ -347,6 +347,46 @@ if system/platform = 'Windows [
 			port? close p
 			not error? try [delete %12345]
 		]
+	--test-- "skip/at on file port"
+		write %12345 "12345"
+		p: open/read/seek %12345
+		;@@ https://github.com/Oldes/Rebol-issues/issues/2506
+		--assert all [
+			1 = index? head p
+			3 = index? skip p 2
+			5 = index? skip p 2
+			6 = index? tail p
+			6 = index? skip p 20
+			1 = index? skip head p -10
+			1 = index? back head p  
+			1 = index? back back head p
+		]
+		--assert all [
+			6 = index? tail p
+			6 = index? at p 20
+			2 = index? at p 2
+			2 = index? at p 2
+			1 = index? at p 0
+			1 = index? at p -10
+		]
+		--assert all [
+			6 = index? tail p
+			6 = index? atz p 20
+			3 = index? atz p 2
+			3 = index? atz p 2
+			1 = index? atz p 0
+			1 = index? atz p -10
+		]
+		--assert all [
+			5 = indexz? tail p
+			5 = indexz? atz p 20
+			2 = indexz? atz p 2
+			2 = indexz? atz p 2
+			0 = indexz? atz p 0
+			0 = indexz? atz p -10
+		]
+		close p
+		delete %12345
 
 	--test-- "CLEAR file port"
 		;@@ https://github.com/Oldes/Rebol-issues/issues/812