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+# LVTL-X: a compact and fair VTL-2 port to POSIX AWK
+
+## About
+
+LVTL-X is a port of the [VTL-2](https://rosettacode.org/wiki/Category:VTL-2) retro programming language interpreter to the POSIX AWK runtime. Unlike the previous effort, LVTL-W, that only worked in BusyBox and GAWK, this version works in any POSIX-compliant environment. On top of that, LVTL-X is extremely compact and takes under 100 SLOC of portable and readable AWK code.
+
+## Usage
+
+The easiest way is to run the `./lvtl` shell wrapper, either stand-alone or with a program file path to preload. You can also specify a non-default AWK interpreter to use with the `AWK` environment variable for the wrapper to use.
+
+Alternatively, you can run the LVTL-X script directly like this:
+
+```sh
+POSIXLY_CORRECT=1 awk -f lvtl-x.awk -- [/path/to/your/vtl/program.vtl] -
+```
+
+If running without a wrapper and specifying a program to preload, don't forget to put another `-` after it.
+
+## Differences from the Altair original
+
+LVTL-X stands out from other LVTL versions by being closer to the original VTL-2, but still has some important differences that shouldn't be ignored:
+
+- both LF and CRLF are accepted (but not saved into RAM)
+- only LFs are printed instead of CRLFs
+- maximum line length is not enforced
+- all whitespace after the line number is fully ignored
+- any whitespace within expressions is also insignificant
+- parentheses are NOT auto-closed at the end of the statement
+- only 26 characters (A-Z) are valid generic variable names
+- supports all standard VTL-2 binary operators: +, -, *, /, =, >, < 
+- jumps to itself (like 10 #=10) are prohibited and counted as nops
+- no file I/O (only the option to preload VTL code from a file)
+- self-modifying programs won't work correctly
+
+Nevertheless, any program that's valid on LVTL-X will run on the original VTL-2/02 implementations too.
+
+## Credits
+
+Created by Luxferre in 2026, released into the public domain with no warranties.
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+#!/bin/sh
+# LVTL-X wrapper/loader
+[ -z "$AWK" ] && AWK=awk
+POSIXLY_CORRECT=1 $AWK -f lvtl-x.awk -- "$1" -
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+# LVTL-X: port of VTL-2 to POSIX AWK
+#
+# Usage: POSIXLY_CORRECT=1 awk -f lvtl-x.awk [prog.vtl -]
+# (don't forget the - after the program or LVTL will exit upon preloading)
+# 
+# Any program valid for LVTL-X will run on the original VTL-2 and VTL02 too
+#
+# Differences from the original VTL-2 for Altair:
+# - both LF and CRLF are accepted (but not saved into RAM)
+# - only LFs are printed instead of CRLFs
+# - maximum line length is not enforced
+# - all whitespace after the line number is fully ignored
+# - any whitespace within expressions is also insignificant
+# - parentheses are NOT auto-closed at the end of the statement
+# - only 26 characters (A-Z) are valid generic variable names
+# - supports all standard VTL-2 binary operators: +, -, *, /, =, >, < 
+# - jumps to itself (like 10 #=10) are prohibited and counted as nops
+# - no file I/O (only the option to preload VTL code from a file)
+# - self-modifying programs won't work correctly
+#
+# Created by Luxferre in 2026 (based on LVTL-W from 2023), released into public domain
+
+function err(m) {print "Fatal error: " m; exit 1}
+
+# read a generic/system variable
+function getvar(varname,    c, cmd) {
+  if(varname == "$") {
+    cmd = "dd bs=1 count=1 2>/dev/null | od -An -t u1"
+    c = (cmd | getline c) > 0 ? int(c) : 0
+    close(cmd); return c
+  }
+  if(varname == "?") { getline c; return evalexpr(c) }
+  if(varname == "'") return int(rand()*65536)
+  return (varname in VARS) ? VARS[varname] : 0
+}
+
+# set a generic/system variable
+function setvar(varname, value,    qi) {
+  if(varname == "$") printf("%c", value % 256)
+  else if(varname == "?") {
+    if(VARS["\""]) {
+      qi = index(value, "\"")
+      printf("%s%s", substr(value, 1, qi - 1), substr(value, qi+1, 1) == ";" ? "" : "\n")
+      VARS["\""] = 0
+    } else printf("%d", value % 65536)
+  } else if(varname in VARS) {
+    value %= 65536; if(varname == "#" && value > 0) VARS["!"] = VARS["#"] + 1
+    VARS[varname] = value
+  } else err("invalid variable name!")
+}
+
+# strict LTR evaluator
+function evalexpr(expr,    tkn, i, l, opex, acc, cop, oprnd, subs) {
+  opex = 1; l = length(expr)
+  for(i=1; i<=l; i++) {
+    tkn = substr(expr, i, 1)
+    if(tkn == ")") break
+    if(tkn == " " || tkn == "\t") continue
+    if(opex = 1 - opex) {
+      if(index("+-*/=<>&!#", tkn)) cop = tkn
+      else err("unexpected binary operator at " VARS["#"])
+    } else {
+      if(tkn == "\"") { VARS[tkn] = 1; return substr(expr, i+1) }
+      if(tkn == ":" || tkn == "(") {
+        subs = evalexpr(substr(expr, i+1)); oprnd = (tkn == ":") ? SCRATCH[subs] : subs
+        i += PROCCHARS
+      } else if(index("0123456789", tkn)) {
+        oprnd = int(substr(expr, i)); i += length(oprnd) - 1
+      } else oprnd = getvar(tkn)
+      oprnd %= 65536
+      if(cop == 0) acc = oprnd
+      else if(cop == "+") acc += oprnd
+      else if(cop == "-") acc -= oprnd
+      else if(cop == "*") acc *= oprnd
+      else if(cop == "/" && oprnd) { VARS["%"] = acc % oprnd; acc = int(acc/oprnd) }
+      else if(cop == "=") acc = (acc == oprnd)
+      else if(cop == ">") acc = (acc >= oprnd)
+      else if(cop == "<") acc = (acc < oprnd)
+      acc = (acc % 65536 + 65536) % 65536
+    }
+  }
+  PROCCHARS = i; return acc
+}
+
+# run the statement (iterative implementation)
+function exec_stmt(stmt, ln,    ei, lhs, rhs, vn, lcache) {
+  while(stmt != "") {
+    if(!(ei = index(stmt, "="))) return
+    if((lhs = substr(stmt, 1, ei - 1)) == "" || (rhs = substr(stmt, ei + 1)) == "") return
+    ei = evalexpr(rhs); vn = substr(lhs, 1, 1)
+    if(vn == ":") SCRATCH[evalexpr(substr(lhs, 2))] = ei
+    else setvar(vn, ei)
+    lcache = ln; ln = VARS["#"]
+    if((!ln && lcache) || (ln > 0 && ln == lcache)) ln = lcache + 1
+    if(ln > 0) {
+      for(ei=ln; ei<65536; ei++) if(ei in PROGLINES) {ln = ei; break}
+      VARS["#"] = ln = (ei >= 65536) ? 0 : ln; stmt = PROGLINES[ln]
+    } else stmt = ""
+  }
+}
+
+BEGIN {
+  srand()
+  for(i=65; i<=90; i++) VARS[sprintf("%c", i)] = 0
+  VARS["!"] = VARS["\""] = VARS["#"] = VARS["%"] = VARS["&"] = 0
+  VARS["*"] = 65535
+  print "LVTL-X by Luxferre\nPress Ctrl+C to exit\n\nOK"
+}
+
+{
+  ln = int($1) % 65536; if(NF > 1) $1 = ""
+  gsub(/^[[:blank:]]+|[[:blank:]]+$/, "")
+  if(ln > 0) {
+    if($0 == "" || $0 == ln) delete PROGLINES[ln]
+    else PROGLINES[ln] = toupper($0)
+  } else {
+    if($0 == "0") {
+      for(i=1; i<65536; i++) if(i in PROGLINES) print i, PROGLINES[i]
+    } else exec_stmt(toupper($0), 0)
+    print "\nOK"
+  }
+}