diff --git a/.gitignore b/.gitignore
index 94c1b10..56c7346 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,3 +2,5 @@ shroom_server
 shrooms.db
 auth_secret
 __pycache__/
+
+test_float_parse
diff --git a/test_float_parse.cc b/test_float_parse.cc
new file mode 100644
index 0000000..c7f7e95
--- /dev/null
+++ b/test_float_parse.cc
@@ -0,0 +1,133 @@
+#include <optional>
+#include <iostream>
+
+bool parseFloat(const char* buf, int len, float* v) {
+  float tmp = 0.0f;
+  float factor = 0.1f;
+  int exponent = 0;
+  bool is_negative = false, has_digit = false;
+  bool using_exponent = false, has_exponent = false;
+  bool exp_is_negative = false;
+
+  enum State {
+    kPredecimal,
+    kPostdecimal, // after the decimal point
+    kExponent, // after the e
+    kExponentPostsign, // after the +/-
+  };
+  State state = kPredecimal;
+
+  for (int i = 0; i < len; i++) {
+    switch (state) {
+      case kPredecimal:
+        if (i == 0 && buf[i] == '-') {
+          is_negative = true;
+        } else if (buf[i] >= '0' && buf[i] <= '9') {
+          has_digit = true;
+          tmp = 10.0f * tmp + (buf[i] - '0');
+        } else if (buf[i] == '.') {
+          state = kPostdecimal;
+        } else if (buf[i] == 'e') {
+          state = kExponent;
+          using_exponent = true;
+        } else {
+          return false;
+        }
+        break;
+      case kPostdecimal:
+        if (buf[i] >= '0' && buf[i] <= '9') {
+          has_digit = true;
+          tmp = tmp + factor * (buf[i] - '0');
+          factor *= 0.1f;
+        } else if (buf[i] == 'e') {
+          state = kExponent;
+          using_exponent = true;
+        } else {
+          return false;
+        }
+        break;
+      case kExponent:
+        if (buf[i] == '+') {
+          state = kExponentPostsign;
+        } else if (buf[i] == '-') {
+          exp_is_negative = true;
+          state = kExponentPostsign;
+        } else if (buf[i] >= '0' && buf[i] <= '9') {
+          has_exponent = true;
+          exponent = (buf[i] - '0');
+          state = kExponentPostsign;
+        } else {
+          return false;
+        }
+        break;
+      case kExponentPostsign:
+        if (buf[i] >= '0' && buf[i] <= '9') {
+          has_exponent = true;
+          exponent = 10*exponent + (buf[i] - '0');
+        } else {
+          return false;
+        }
+        break;
+    }
+  }
+
+  if (!has_digit) return false;
+  if (using_exponent && !has_exponent) return false;
+
+  if (is_negative) {
+    tmp = -tmp;
+  }
+  if (has_exponent) {
+    if (exp_is_negative) {
+      for (int i = 0; i < exponent; i++) {
+        tmp *= 0.1f;
+      }
+    } else {
+      for (int i = 0; i < exponent; i++) {
+        tmp *= 10.f;
+      }
+    }
+  }
+
+  *v = tmp;
+  return true;
+}
+
+std::optional<float> parseWrapper(const char* buf, int len) {
+  float tmp;
+  if (parseFloat(buf, len, &tmp)) {
+    return tmp;
+  } else {
+    return std::nullopt;
+  }
+}
+
+#define ASSERT_VALID_MATCHES(s) \
+  do { \
+    auto tmp = parseWrapper(#s, sizeof(#s) - 1); \
+    if (!tmp) { \
+      std::cout << "failed to parse " << s << std::endl; \
+      return -1; \
+    } \
+    float err = (*tmp - s) / (s + 1e-12); \
+    if (err < 0) err = -err; \
+    if (err > 1e-6) { \
+      std::cout << "error out of range for " << #s << " err = " << err << ", parsed = " << *tmp << ", expected = " << s << std::endl; \
+    } \
+  } while (0)
+
+int main() {
+  ASSERT_VALID_MATCHES(1.);
+  ASSERT_VALID_MATCHES(0.1);
+  ASSERT_VALID_MATCHES(-0.1);
+  ASSERT_VALID_MATCHES(-0.1e3);
+  ASSERT_VALID_MATCHES(-0.1e-3);
+  ASSERT_VALID_MATCHES(1.2e-3);
+  ASSERT_VALID_MATCHES(.2e-3);
+  ASSERT_VALID_MATCHES(-.3e-4);
+  ASSERT_VALID_MATCHES(-.3e4);
+  ASSERT_VALID_MATCHES(-.0003342);
+  ASSERT_VALID_MATCHES(-.0073342e12);
+  ASSERT_VALID_MATCHES(3.141592);
+  return 0;
+}