From 63be80aee1824327e51dde0cdcd1e3d014d4f707 Mon Sep 17 00:00:00 2001
From: yjianpen <javieryu4229@gmail.com>
Date: Sat, 23 Aug 2025 17:23:17 -0700
Subject: [PATCH] Adding the most popular interview question: implementing lora
 from scratch!

---
 Problems/140_low_rank_adaptation/learn.md    | 47 ++++++++++++++++++++
 Problems/140_low_rank_adaptation/solution.py | 32 +++++++++++++
 2 files changed, 79 insertions(+)
 create mode 100644 Problems/140_low_rank_adaptation/learn.md
 create mode 100644 Problems/140_low_rank_adaptation/solution.py

diff --git a/Problems/140_low_rank_adaptation/learn.md b/Problems/140_low_rank_adaptation/learn.md
new file mode 100644
index 00000000..afd84213
--- /dev/null
+++ b/Problems/140_low_rank_adaptation/learn.md
@@ -0,0 +1,47 @@
+
+# Learn Section
+
+### Writing Mathematical Expressions with LaTeX
+
+This editor supports LaTeX for rendering mathematical equations and expressions. Here's how you can use it:
+
+1. **Inline Math**: 
+   - Wrap your expression with single `$` symbols.
+   - Example: `$E = mc^2$` → Renders as: ( $E = mc^2$ )
+
+2. **Block Math**:
+   - Wrap your expression with double `$$` symbols.
+   - Example: 
+     ```
+     $$
+     \int_a^b f(x) \, dx
+     $$
+     ```
+     Renders as:
+     $$
+     \int_a^b f(x) \, dx
+     $$
+
+3. **Math Functions**:
+   - Use standard LaTeX functions like `\frac`, `\sqrt`, `\sum`, etc.
+   - Examples:
+     - `$\frac{a}{b}$` → ( $\frac{a}{b}$ )
+     - `$\sqrt{x}$` → ( $\sqrt{x}$ )
+
+4. **Greek Letters and Symbols**:
+   - Use commands like `\alpha`, `\beta`, etc., for Greek letters.
+   - Example: `$\alpha + \beta = \gamma$` → ( $\alpha + \beta = \gamma$ )
+
+5. **Subscripts and Superscripts**:
+   - Use `_{}` for subscripts and `^{}` for superscripts.
+   - Examples:
+     - `$x_i$` → ( $x_i$ )
+     - `$x^2$` → ( $x^2$ )
+
+6. **Combined Examples**:
+   - `$\sum_{i=1}^n i^2 = \frac{n(n+1)(2n+1)}{6}$`
+     Renders as:
+     $\sum_{i=1}^n i^2 = \frac{n(n+1)(2n+1)}{6}$
+
+Feel free to write your own mathematical expressions, and they will be rendered beautifully in the preview!
+    
\ No newline at end of file
diff --git a/Problems/140_low_rank_adaptation/solution.py b/Problems/140_low_rank_adaptation/solution.py
new file mode 100644
index 00000000..86d570d8
--- /dev/null
+++ b/Problems/140_low_rank_adaptation/solution.py
@@ -0,0 +1,32 @@
+class LoRALinear(nn.Module):
+    def __init__(self, original_layer_shape: tuple, rank: int, alpha: float, init_scaling_factor:float, random_seed:int):
+        super().__init__()
+        # Freeze the original layer's weights
+        # Extract in_features and out_features from the shape tuple
+        in_features, out_features = original_layer_shape
+        self.original_layer = torch.nn.Linear(in_features, out_features)
+        self.original_layer.requires_grad_(False)
+
+        # Initialize the low-rank matrices A and B
+        g = torch.Generator()
+        g.manual_seed(random_seed)
+        self.lora_A = nn.Parameter(torch.randn(in_features, rank, generator=g) * init_scaling_factor)
+        print("lora A", self.lora_A)
+        self.lora_B = nn.Parameter(torch.zeros(rank, out_features))
+        print("lora B", self.lora_B)
+
+        # The scaling factor
+        self.scaling = alpha / rank
+
+    def forward(self, x):
+        # Original output
+        x = torch.tensor(x)
+        original_output = self.original_layer(x)
+
+        # LoRA update
+        # (x @ self.lora_A) performs matrix multiplication of input x with matrix A
+        # (x @ self.lora_A @ self.lora_B) is the full low-rank update
+        lora_update = x @ self.lora_A @ self.lora_B
+
+        # Final output is the sum of the original and the scaled LoRA update
+        return original_output + lora_update * self.scaling
\ No newline at end of file