redid comparison without addition or subtraction due to realization that doing them relative could lead to cycle

integer.cpp

@@ -2,6 +2,7 @@
 #include <cassert>
 #include <cstddef>
 #include <iostream>
+#include <limits>
 #include <sstream>
 #include <string>
 #define todo(msg)                                                              \
@@ -59,8 +60,68 @@ public:
     isNegative = rhs.isNegative;
     return *this;
   }
+  void expand() {
+    byte *new_buf = new byte[size + 1];
+    std::copy(bytes, bytes + size, new_buf);
+    size++;
+    delete[] bytes;
+    bytes = new_buf;
+  }
+  std::size_t true_size() const {
+    std::size_t ret = size;
+    if (bytes == nullptr) {
+      return 0;
+    }
+    while (bytes[ret - 1] == 0 && ret > 1) {
+      ret--;
+    }
+    return ret;
+  }
+  friend bool abs_lt(Integer const &lhs, Integer const &rhs) {
+    std::size_t true_lhs_size = lhs.true_size();
+    std::size_t true_rhs_size = rhs.true_size();
+    if (true_lhs_size != true_rhs_size) {
+      return true_lhs_size < true_rhs_size;
+    }
+    if (true_lhs_size == 0) {
+      return false;
+    }
+
+    auto idx = true_lhs_size - 1;
+    while (idx >= 0) {
+      if (lhs.bytes[idx] != rhs.bytes[idx]) {
+        return lhs.bytes[idx] < rhs.bytes[idx];
+      }
+    }
+    return false;
+  }
   friend Integer &operator+=(Integer &lhs, Integer const &rhs) {
-    todo("+=");
+    // TODO: negative numbers
+    bool overflow = false;
+    for (std::size_t i = 0; i < std::min(lhs.size, rhs.size); i++) {
+      if (std::numeric_limits<byte>::max() - lhs.bytes[i] <= rhs.bytes[i]) {
+        overflow = true;
+      } else if (std::numeric_limits<byte>::max() - lhs.bytes[i] -
+                     (rhs.bytes[i]) <=
+                 (overflow ? 1 : 0)) {
+        overflow = true;
+      }
+      lhs.bytes[i] += rhs.bytes[i] + (overflow ? 1 : 0);
+    }
+    if (overflow) {
+      std::size_t idx = std::min(lhs.size, rhs.size);
+      if (idx == lhs.size) {
+        lhs.expand();
+      }
+      lhs.bytes[idx]++;
+      if (lhs.bytes[idx] == 0) {
+        idx++;
+        if (idx == lhs.size) {
+          lhs.expand();
+        }
+        lhs.bytes[idx]++;
+      }
+    }
     return lhs;
   }
   friend Integer &operator-=(Integer &lhs, Integer const &rhs) {
@@ -129,17 +190,19 @@ public:
     return ret;
   }
   friend bool operator<(Integer const &lhs, Integer const &rhs) {
-    Integer tmp = (lhs - rhs);
-    return tmp.isNegative && tmp != 0;
+    if (lhs.isNegative != rhs.isNegative) {
+      return lhs.isNegative;
+    } else {
+      return abs_lt(lhs, rhs);
+    }
   }
   friend bool operator>(Integer const &lhs, Integer const &rhs) {
-    Integer tmp = (rhs - lhs);
-    return tmp.isNegative && tmp != 0;
+    return rhs < lhs;
   }
   friend bool operator==(Integer const &lhs, Integer const &rhs) {
-    todo("need to actually do a check for the case that one or both of them is "
-         "0");
-    return (rhs - lhs) == 0;
+    auto l_true = lhs.true_size();
+    return l_true == rhs.true_size() &&
+           std::equal(lhs.bytes, lhs.bytes + l_true, rhs.bytes);
   }
 
   friend std::ostream &operator<<(std::ostream &lhs, Integer const &rhs) {