Merge pull request #28 from Lilit0x/functions/substring

SUBSTRING String Function: #13

crates/gitql-ast/src/function.rs

@@ -29,6 +29,7 @@ lazy_static! {
         map.insert("left", text_left);
         map.insert("datalength", text_datalength);
         map.insert("char", text_char);
+        map.insert("substring", text_substring);
 
         // Date functions
         map.insert("current_date", date_current_date);
@@ -133,6 +134,13 @@ lazy_static! {
                 result: DataType::Text,
             },
         );
+        map.insert(
+            "substring",
+            Prototype {
+                parameters: vec![DataType::Text, DataType::Number, DataType::Number],
+                result: DataType::Text,
+            },
+        );
 
         // Date functions
         map.insert(
@@ -242,6 +250,23 @@ fn text_char(inputs: Vec<Value>) -> Value {
     return Value::Text("".to_string());
 }
 
+fn text_substring(inputs: Vec<Value>) -> Value {
+    let text = inputs[0].as_text();
+    //according to the specs, a stirng starts at position 1.
+    //but in Rust, the index of a string starts from 0
+    let start = inputs[1].as_number() as usize - 1;
+    let length = inputs[2].as_number();
+
+    if start > text.len() || length > text.len() as i64 {
+        return Value::Text(text);
+    }
+    if length < 0 {
+        return Value::Text("".to_string());
+    }
+
+    return Value::Text(text[start..(start + length as usize)].to_string());
+}
+
 // Date functions
 
 fn date_current_date(_inputs: Vec<Value>) -> Value {

docs/function/functions.md

@@ -18,6 +18,7 @@ note that all functions names are case-insensitive.
 | LEFT       | Text, Number | Text   | Extracts a number of characters from a string (starting from left). |
 | DATALENGTH | Text         | Number | Returns the number of bytes used to represent an expression.        |
 | CHAR       | Number       | Text   | Returns the character based on the ASCII code.                      |
+| SUBSTRING  | Text, Number, Number | Text | Extracts some characters from a string.                       |
 
 ### String functions samples
 
@@ -32,6 +33,7 @@ SELECT name, ASCII(name) AS firstCharAscii FROM commits
 SELECT LEFT("AmrDeveloper", 3) AS extract
 SELECT DATALENGTH("AmrDeveloper") as bytelength
 SELECT CHAR(345) AS code
+SELECT name, SUBSTRING(name, 1, 5) AS extract FROM commits
 ```
 
 ### Date functions