Write documentation for AST Builder lpad & rpad (gluesql#1434)

docs/docs/ast-builder/functions/text/padding.md

@@ -1,5 +1,79 @@
-# Padding 
-## Todo
-    - LPAD: Left-pads a string with a specified string up to a certain length.
-    - RPAD: Right-pads a string with a specified string up to a certain length.
+# Padding
 
+The AST Builder API in GlueSQL allows you to execute lpad and rpad functions for text padding.
+
+## lpad
+
+`lpad` returns the string with leading space if the length of the string is less than the specified length.
+
+```rs
+lpad<'a, T: Into<ExprNode<'a>>>(expr: T, len: usize, fill: Option<String>) -> ExprNode<'a>
+```
+
+## rpad
+
+`rpad` returns the string with trailing space if the length of the string is less than the specified length.
+
+```rs
+rpad<'a, T: Into<ExprNode<'a>>>(expr: T, len: usize, fill: Option<String>) -> ExprNode<'a>
+```
+
+## Examples
+
+In these examples, the LPAD and RPAD functions should return matching values.
+
+```rs
+use {
+    crate::*,
+    gluesql_core::{
+        ast_builder::{function as f, *},
+        prelude::Value::{Null, Str},
+    },
+};
+
+test_case!(padding, {
+    let glue = get_glue!();
+
+    let actual = values(vec![
+        vec![f::lpad("'hello'", 10, None), f::rpad("'hello'", 10, None)],
+        vec![
+            f::lpad("'hello'", 10, Some("'ab'".into())),
+            f::rpad("'hello'", 10, Some("'ab'".into())),
+        ],
+        vec![f::lpad("'hello'", 3, None), f::rpad("'hello'", 3, None)],
+        vec![
+            f::lpad("'hello'", 3, Some("'ab'".into())),
+            f::rpad("'hello'", 3, Some("'ab'".into())),
+        ],
+        vec![f::lpad("NULL", 5, None), f::rpad("NULL", 5, None)],
+    ])
+    .alias_as("Sub")
+    .select()
+    .project("column1 AS lpaded")
+    .project("column2 AS rpaded")
+    .execute(glue)
+    .await;
+    let expected = Ok(select_with_null!(
+        lpaded                       | rpaded;
+        Str("     hello".to_owned())   Str("hello     ".to_owned());
+        Str("ababahello".to_owned())   Str("helloababa".to_owned());
+        Str("hel".to_owned())          Str("hel".to_owned());
+        Str("hel".to_owned())          Str("hel".to_owned());
+        Null                           Null
+    ));
+    assert_eq!(
+        actual, expected,
+        "lpad and rpad should pad the string with given length"
+    );
+});
+```
+
+```
+| lpaded       | rpaded       |
+| ------------ | ------------ |
+| '     hello' | 'hello     ' |
+| 'ababahello' | 'helloababa' |
+| 'hel'        | 'hel'        |
+| 'hel'        | 'hel'        |
+| Null         | Null         |
+```