Fix off-by-one error when printing numbers in debug output

Based on the fix by @ginnyTheCat in
#392.

Author: joto

include/osmium/io/detail/debug_output_format.hpp

@@ -106,6 +106,14 @@ namespace osmium {
 
             }; // struct debug_output_options
 
+            // Space needed for printing numbers from 0 to num - 1
+            int print_width(std::size_t num) noexcept {
+                if (num <= 10) {
+                    return 1;
+                }
+                return static_cast<int>(std::log10(num - 1)) + 1;
+            }
+
             /**
              * Writes out one buffer with OSM data in Debug format.
              */
@@ -392,7 +400,7 @@ namespace osmium {
                         *m_out += " (open)\n";
                     }
 
-                    const int width = static_cast<int>(std::log10(way.nodes().size())) + 1;
+                    const int width = print_width(way.nodes().size());
                     int n = 0;
                     for (const auto& node_ref : way.nodes()) {
                         write_diff();
@@ -427,7 +435,7 @@ namespace osmium {
                     output_int(relation.members().size());
                     *m_out += '\n';
 
-                    const int width = static_cast<int>(std::log10(relation.members().size())) + 1;
+                    const int width = print_width(relation.members().size());
                     int n = 0;
                     for (const auto& member : relation.members()) {
                         write_diff();
@@ -485,7 +493,7 @@ namespace osmium {
                         output_int(changeset.num_comments());
                         *m_out += '\n';
 
-                        const int width = static_cast<int>(std::log10(changeset.num_comments())) + 1;
+                        const int width = print_width(changeset.num_comments());
                         int n = 0;
                         for (const auto& comment : changeset.discussion()) {
                             write_counter(width, n++);

test/CMakeLists.txt

@@ -180,6 +180,7 @@ add_unit_test(io test_nocompression)
 add_unit_test(io test_output_utils)
 add_unit_test(io test_file_seek)
 add_unit_test(io test_string_table)
+add_unit_test(io test_print_width ENABLE_IF ${ZLIB_FOUND} LIBS ${ZLIB_LIBRARIES})
 
 add_unit_test(io test_bzip2 ENABLE_IF ${BZIP2_FOUND} LIBS ${BZIP2_LIBRARIES})
 add_unit_test(io test_gzip ENABLE_IF ${ZLIB_FOUND} LIBS ${ZLIB_LIBRARIES})

test/t/io/test_print_width.cpp

@@ -0,0 +1,19 @@
+#include "catch.hpp"
+
+#include <osmium/io/detail/debug_output_format.hpp>
+
+TEST_CASE("Calculate width of a number") {
+    REQUIRE(osmium::io::detail::print_width(0) == 1);
+    REQUIRE(osmium::io::detail::print_width(1) == 1);
+    REQUIRE(osmium::io::detail::print_width(2) == 1);
+    REQUIRE(osmium::io::detail::print_width(9) == 1);
+    REQUIRE(osmium::io::detail::print_width(10) == 1); // 0 .. 9
+    REQUIRE(osmium::io::detail::print_width(11) == 2);
+    REQUIRE(osmium::io::detail::print_width(42) == 2);
+    REQUIRE(osmium::io::detail::print_width(99) == 2);
+    REQUIRE(osmium::io::detail::print_width(100) == 2); // 0 .. 99
+    REQUIRE(osmium::io::detail::print_width(101) == 3);
+    REQUIRE(osmium::io::detail::print_width(999) == 3);
+    REQUIRE(osmium::io::detail::print_width(1000) == 3); // 0 .. 100
+    REQUIRE(osmium::io::detail::print_width(1001) == 4);
+}