From 6d43c1316cf2e7f438c6ca73a43b63d063d7029f Mon Sep 17 00:00:00 2001
From: Egor Tensin <Egor.Tensin@gmail.com>
Date: Tue, 31 Dec 2019 20:51:34 +0300
Subject: parser: support unary +

---
 server/parser/parser.hpp   | 24 ++++++++++++++++++------
 test/unit_tests/parser.cpp | 23 ++++++++++++++++++-----
 2 files changed, 36 insertions(+), 11 deletions(-)

diff --git a/server/parser/parser.hpp b/server/parser/parser.hpp
index 7d6f31d..fb2a5ff 100644
--- a/server/parser/parser.hpp
+++ b/server/parser/parser.hpp
@@ -36,7 +36,7 @@ public:
 
 private:
     double exec_expr() {
-        return exec_expr(exec_primary(), parser::BinaryOp::min_precedence());
+        return exec_expr(exec_factor(), parser::BinaryOp::min_precedence());
     }
 
     double exec_expr(double lhs, unsigned min_prec) {
@@ -45,7 +45,7 @@ private:
             const auto prev_prec = prev.get_precedence();
 
             m_lexer.drop_token();
-            auto rhs = exec_primary();
+            auto rhs = exec_factor();
 
             for (op = peek_operator(); op.has_value(); op = peek_operator()) {
                 const auto next = *op;
@@ -77,16 +77,28 @@ private:
         return parser::BinaryOp::from_token(*token);
     }
 
-    double exec_primary() {
+    double exec_factor() {
         if (!m_lexer.has_token()) {
-            throw ParserError{"expected '-', '(' or a number"};
+            throw ParserError{"expected '-', '+', '(' or a number"};
         }
 
         using Type = lexer::Token::Type;
 
         if (m_lexer.drop_token_of_type(Type::MINUS).has_value()) {
-            return -exec_primary();
+            return -exec_factor();
         }
+        if (m_lexer.drop_token_of_type(Type::PLUS).has_value()) {
+            return exec_factor();
+        }
+        return exec_atom();
+    }
+
+    double exec_atom() {
+        if (!m_lexer.has_token()) {
+            throw ParserError{"expected '-', '+', '(' or a number"};
+        }
+
+        using Type = lexer::Token::Type;
 
         if (m_lexer.drop_token_of_type(Type::LEFT_PAREN).has_value()) {
             const auto inner = exec_expr();
@@ -100,7 +112,7 @@ private:
             return token.value().as_number();
         }
 
-        throw ParserError{"expected '-', '(' or a number"};
+        throw ParserError{"expected '-', '+', '(' or a number"};
     }
 
     Lexer m_lexer;
diff --git a/test/unit_tests/parser.cpp b/test/unit_tests/parser.cpp
index 37d9e0a..594e162 100644
--- a/test/unit_tests/parser.cpp
+++ b/test/unit_tests/parser.cpp
@@ -24,20 +24,31 @@ namespace {
 namespace exec::valid {
 
 const std::vector<std::string_view> input{
+    // Constants and unary operators:
     "1",
+    "-1",
+    "--1",
+    "+--+-2",
+    // Basic binary operators:
     " 1 + 2 ",
     " 2 * 1 + 3 ",
     " 2 * (1 + 3) ",
     " 2 * (1 + 3 * (1 - -3)) ",
-    " -2 * ---- (3 + -100e-1)  ", // Looks weird, but also works in e.g. Python
+    // Looks weird, but also works in e.g. Python:
+    " -2 * -+--- (3 + -100e-1)  ",
+    // Power operator is right-associative:
     "2 ^ 3 ^ 3",
-    "(2 ^ 3) ^ 3", // Power operator is right-associative
+    "(2 ^ 3) ^ 3",
+    // Power operator has higher precedence than the unary minus:
     "(.5 ^ -1) ^ 4",
-    ".5 ^ -1 ^ 4", // Power operator has higher precedence than the unary minus
+    ".5 ^ -1 ^ 4",
 };
 
 const std::vector<double> expected{
     1,
+    -1,
+    1,
+    -2,
     3,
     5,
     8,
@@ -55,14 +66,16 @@ namespace exec::invalid {
 
 const std::vector<std::string_view> input{
     "",
+    // Missing operand:
     " 1 + ",
+    // Unmatched parentheses:
     " 2 * (1 + 3 ",
     " 2 * (1 + 3) )",
 };
 
 const std::vector<std::string> error_msg{
-    "server error: parser error: expected '-', '(' or a number",
-    "server error: parser error: expected '-', '(' or a number",
+    "server error: parser error: expected '-', '+', '(' or a number",
+    "server error: parser error: expected '-', '+', '(' or a number",
     "server error: parser error: missing closing ')'",
     "server error: parser error: expected a binary operator",
 };
-- 
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