[flang] External I/O runtime work, repackaged (part 2)

[klausler]

Summary

Clean up the input editing path so external input works better
when combined with further changes. List-directed input needed
to allow for advancement to following records.

flang/runtime/edit-input.cpp

 //===-- runtime/edit-input.cpp ----------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "edit-input.h"
 #include "flang/Common/real.h"
 #include "flang/Common/uint128.h"
 #include <algorithm>
 
 namespace Fortran::runtime::io {
 
+static std::optional<char32_t> PrepareInput(
+    IoStatementState &io, const DataEdit &edit, std::optional<int> &remaining) {
+  remaining.reset();
+  if (edit.descriptor == DataEdit::ListDirected) {
+    io.GetNextNonBlank();
+  } else {
+    if (edit.width.value_or(0) > 0) {
+      remaining = *edit.width;
+    }
+    io.SkipSpaces(remaining);
+  }
+  return io.NextInField(remaining);
+}
+
 static bool EditBOZInput(IoStatementState &io, const DataEdit &edit, void *n,
     int base, int totalBitSize) {
   std::optional<int> remaining;
-  if (edit.width) {
-    remaining = std::max(0, *edit.width);
-  }
-  io.SkipSpaces(remaining);
-  std::optional<char32_t> next{io.NextInField(remaining)};
+  std::optional<char32_t> next{PrepareInput(io, edit, remaining)};
   common::UnsignedInt128 value{0};
   for (; next; next = io.NextInField(remaining)) {
     char32_t ch{*next};
     if (ch == ' ') {
       continue;
     }
     int digit{0};
     if (ch >= '0' && ch <= '1') {
static bool EditBOZInput(IoStatementState &io, const DataEdit &edit, void *n,
   // TODO: check for overflow
   std::memcpy(n, &value, totalBitSize >> 3);
   return true;
 }
 
 // Returns false if there's a '-' sign
 static bool ScanNumericPrefix(IoStatementState &io, const DataEdit &edit,
     std::optional<char32_t> &next, std::optional<int> &remaining) {
-  if (edit.descriptor != DataEdit::ListDirected && edit.width) {
-    remaining = std::max(0, *edit.width);
-  } else {
-    // list-directed, namelist, or (nonstandard) 0-width input editing
-    remaining.reset();
-  }
-  io.SkipSpaces(remaining);
-  next = io.NextInField(remaining);
+  next = PrepareInput(io, edit, remaining);
   bool negative{false};
   if (next) {
     negative = *next == '-';
     if (negative || *next == '+') {
       next = io.NextInField(remaining);
     }
   }
   return negative;
case 'G':
     break;
   default:
     io.GetIoErrorHandler().SignalError(IostatErrorInFormat,
         "Data edit descriptor '%c' may not be used for LOGICAL input",
         edit.descriptor);
     return false;
   }
   std::optional<int> remaining;
-  if (edit.width) {
-    remaining = std::max(0, *edit.width);
-  }
-  io.SkipSpaces(remaining);
-  std::optional<char32_t> next{io.NextInField(remaining)};
+  std::optional<char32_t> next{PrepareInput(io, edit, remaining)};
   if (next && *next == '.') { // skip optional period
     next = io.NextInField(remaining);
   }
   if (!next) {
     io.GetIoErrorHandler().SignalError("Empty LOGICAL input field");
     return false;
   }
   switch (*next) {