schemagen: parse integer fallback exactly for decimal/exponent forms

Replace the double-based fallback in generated integer slots with a
string-to-int64 parser that handles decimal points and exponents
without losing precision near the int64 boundaries.

The old path used std::from_chars<double> and compared against
±9223372036854775808.0, which rounds the int64 max and min so that
valid values are rejected and out-of-range negatives are accepted.

The new helper:
- Parses sign, integer part, optional fraction, and optional exponent.
- Strips trailing zeros to cancel fractional places.
- Rejects non-integral values and overflow using exact uint64_t
  arithmetic.

Adds regression tests covering root integer and object-field integer
boundary values, including the cases from issue #19.

contrib/schemagen/test_schemagen.py

@@ -8,6 +8,7 @@ import shutil
 import subprocess
 import sys
 import tempfile
+import textwrap
 import unittest
 
 SCRIPT = os.path.join(os.path.dirname(__file__), "weaseljson_schemagen.py")
@@ -242,5 +243,200 @@ class SchemagenAdditionalPropertiesTest(unittest.TestCase):
         self.assertEqual(rc, 0, msg=stderr)
 
 
+class SchemagenIntegerBoundaryTest(unittest.TestCase):
+    """Regression tests for issue #19: integer slot parsing near int64 boundaries."""
+
+    def setUp(self):
+        self.repo_root = os.path.dirname(
+            os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+        )
+        self.include_dir = os.path.join(self.repo_root, "include")
+        self.lib_src = os.path.join(self.repo_root, "src", "lib.cpp")
+        self.compiler = shutil.which("c++")
+
+    def _compile_harness(self, tmpdir, schema, harness):
+        schema_path = os.path.join(tmpdir, "schema.json")
+        with open(schema_path, "w") as fp:
+            json.dump(schema, fp)
+        header_path = os.path.join(tmpdir, "gen.h")
+        result = subprocess.run(
+            [
+                sys.executable,
+                SCRIPT,
+                schema_path,
+                "-o",
+                header_path,
+                "--namespace",
+                "test_schema",
+            ],
+            capture_output=True,
+            text=True,
+            check=False,
+        )
+        self.assertEqual(result.returncode, 0, msg=result.stderr)
+
+        lib_obj = os.path.join(tmpdir, "lib.o")
+        comp_lib = subprocess.run(
+            [
+                self.compiler,
+                "-std=c++20",
+                "-I",
+                self.include_dir,
+                "-I",
+                os.path.join(self.repo_root, "third_party", "include"),
+                "-I",
+                os.path.join(self.repo_root, "third_party", "valgrind"),
+                "-c",
+                self.lib_src,
+                "-o",
+                lib_obj,
+            ],
+            capture_output=True,
+            text=True,
+            check=False,
+        )
+        self.assertEqual(comp_lib.returncode, 0, msg=comp_lib.stderr)
+
+        cpp_path = os.path.join(tmpdir, "test.cpp")
+        with open(cpp_path, "w") as fp:
+            fp.write(harness)
+
+        exe_path = os.path.join(tmpdir, "test")
+        comp = subprocess.run(
+            [
+                self.compiler,
+                "-std=c++20",
+                "-I",
+                self.include_dir,
+                "-I",
+                tmpdir,
+                lib_obj,
+                cpp_path,
+                "-o",
+                exe_path,
+            ],
+            capture_output=True,
+            text=True,
+            check=False,
+        )
+        self.assertEqual(comp.returncode, 0, msg=comp.stderr)
+        run = subprocess.run([exe_path], capture_output=True, text=True, check=False)
+        self.assertEqual(run.returncode, 0, msg=run.stdout + run.stderr)
+
+    def _build_cases_array(self, name, entries):
+        lines = [
+            f"  struct {name}Case {{ const char *s; WeaselJsonStatus expected; long long v; }};"
+        ]
+        lines.append(f"  {name}Case {name}_cases[] = {{")
+        for s, exp, v in entries:
+            val = f"{v}LL" if isinstance(v, int) else v
+            lines.append(f'    {{ R"({s})", {exp}, {val} }},')
+        lines.append("  };")
+        return "\n".join(lines)
+
+    def test_integer_boundary_root(self):
+        if not self.compiler:
+            self.skipTest("C++ compiler not available")
+        cases = [
+            ("9223372036854775807.0", "WeaselJson_OK", 9223372036854775807),
+            ("9223372036854775807", "WeaselJson_OK", 9223372036854775807),
+            ("9223372036854775806.0", "WeaselJson_OK", 9223372036854775806),
+            ("9223372036854775808", "WeaselJson_REJECT", 0),
+            ("-9223372036854775808", "WeaselJson_OK", "-9223372036854775807LL - 1"),
+            ("-9223372036854775808.0", "WeaselJson_OK", "-9223372036854775807LL - 1"),
+            ("-9223372036854775809", "WeaselJson_REJECT", 0),
+            ("1e3", "WeaselJson_OK", 1000),
+            ("2.0", "WeaselJson_OK", 2),
+            ("0.001", "WeaselJson_REJECT", 0),
+            ("1e-3", "WeaselJson_REJECT", 0),
+            ("1000e-3", "WeaselJson_OK", 1),
+            ("100.0e-2", "WeaselJson_OK", 1),
+            ("123.0", "WeaselJson_OK", 123),
+            ("9e18", "WeaselJson_OK", 9000000000000000000),
+            ("10e18", "WeaselJson_REJECT", 0),
+        ]
+        cases_src = self._build_cases_array("root", cases)
+        harness = textwrap.dedent(
+            f"""
+            #include "gen.h"
+            #include <cstdio>
+            #include <cstring>
+            {cases_src}
+            int main() {{
+              for (const auto \u0026c : root_cases) {{
+                test_schema::RootBuilder b;
+                char buf[512];
+                std::strncpy(buf, c.s, sizeof(buf) - 1);
+                buf[sizeof(buf) - 1] = '\\0';
+                WeaselJsonStatus st = b.feed(buf, std::strlen(buf));
+                st = b.finish();
+                if (st != c.expected) {{
+                  std::printf("root case %s expected %d got %d\\n", c.s, c.expected, st);
+                  return 1;
+                }}
+                if (st == WeaselJson_OK \u0026\u0026 b.take() != c.v) {{
+                  std::printf("root case %s value mismatch\\n", c.s);
+                  return 2;
+                }}
+              }}
+              return 0;
+            }}
+            """
+        )
+        with tempfile.TemporaryDirectory() as tmpdir:
+            self._compile_harness(tmpdir, {"type": "integer"}, harness)
+
+    def test_integer_boundary_object_field(self):
+        if not self.compiler:
+            self.skipTest("C++ compiler not available")
+        cases = [
+            ('{"age":9223372036854775807.0}', "WeaselJson_OK", 9223372036854775807),
+            ('{"age":-9223372036854775809}', "WeaselJson_REJECT", 0),
+            ('{"age":1e3}', "WeaselJson_OK", 1000),
+            ('{"age":2.0}', "WeaselJson_OK", 2),
+            ('{"age":0.001}', "WeaselJson_REJECT", 0),
+            (
+                '{"age":-9223372036854775808.0}',
+                "WeaselJson_OK",
+                "-9223372036854775807LL - 1",
+            ),
+        ]
+        cases_src = self._build_cases_array("object", cases)
+        harness = textwrap.dedent(
+            f"""
+            #include "gen.h"
+            #include <cstdio>
+            #include <cstring>
+            {cases_src}
+            int main() {{
+              for (const auto \u0026c : object_cases) {{
+                test_schema::RootBuilder b;
+                char buf[512];
+                std::strncpy(buf, c.s, sizeof(buf) - 1);
+                buf[sizeof(buf) - 1] = '\\0';
+                WeaselJsonStatus st = b.feed(buf, std::strlen(buf));
+                st = b.finish();
+                if (st != c.expected) {{
+                  std::printf("object case %s expected %d got %d\\n", c.s, c.expected, st);
+                  return 3;
+                }}
+                if (st == WeaselJson_OK \u0026\u0026 b.take().age != c.v) {{
+                  std::printf("object case %s value mismatch\\n", c.s);
+                  return 4;
+                }}
+              }}
+              return 0;
+            }}
+            """
+        )
+        schema = {
+            "type": "object",
+            "properties": {"age": {"type": "integer"}},
+            "required": ["age"],
+        }
+        with tempfile.TemporaryDirectory() as tmpdir:
+            self._compile_harness(tmpdir, schema, harness)
+
+
 if __name__ == "__main__":
     unittest.main()