feat(growth): add weight-for-height calculation

lib/growth/score/weight_for_height.ex

@@ -1,5 +1,95 @@
 defmodule Growth.Score.WeightForHeight do
   @moduledoc """
   Calculate z-score for weight for height.
+
+  This module calculates the z-score for weight relative to height, which is an important
+  indicator for assessing whether a child's weight is appropriate for their height,
+  regardless of age.
+
+  **Limitation**: the memasurements do not differentiate between length and height, and
+  always assume height.
   """
+
+  @behaviour Growth.Score.Scorer
+
+  alias Growth.Score.Scorer
+
+  @impl Scorer
+  @spec measure_name() :: atom()
+  @doc """
+  Name of the measurement used in weight for height indicator.
+  """
+  def measure_name do
+    :weight_for_height
+  end
+
+  @doc """
+  Custom implementation for weight-for-height lookup, as it requires both weight and height.
+
+  This overrides the default implementation in the Scorer module.
+  """
+  @spec lms(Growth.t(), module()) :: [{String.t(), {number(), number(), number()}}]
+  def lms(%Growth{gender: gender, weight: weight, height: height}, _indicator)
+      when is_number(weight) and is_number(height) do
+    # For weight-for-height, we use height as the lookup value instead of age
+    key = {{gender, :height, height}, :_}
+
+    case :ets.match_object(__MODULE__, key) do
+      [{{^gender, _, ^height}, %{l: l, m: m, s: s}} | _] ->
+        [{"height", {l, m, s}}]
+
+      _ ->
+        # Try to find the closest height value
+        find_closest_height(gender, height)
+    end
+  end
+
+  def lms(_growth, _indicator) do
+    []
+  end
+
+  @doc """
+  Find the closest height value in the ETS table when an exact match isn't found.
+  """
+  @spec find_closest_height(Growth.gender(), number()) :: [
+          {String.t(), {number(), number(), number()}}
+        ]
+  def find_closest_height(gender, height) do
+    # Fetch all entries for the given gender and height measurement
+    matcher = [
+      {
+        {{:"$1", :"$2", :"$3"}, :_},
+        [
+          {:andalso,
+           {:andalso, {:andalso, {:==, :"$1", gender}, {:==, :"$2", :height}},
+            {:>, :"$3", height - 1.0}}, {:<, :"$3", height + 1.0}}
+        ],
+        [:"$_"]
+      }
+    ]
+
+    gender_height_entries = :ets.select(__MODULE__, matcher)
+
+    case gender_height_entries do
+      [] ->
+        # No entries found for this gender
+        []
+
+      entries ->
+        # Find the entry with the height closest to the target height
+        closest_entry =
+          entries
+          |> Enum.filter(fn {{_, _, entry_height}, _lms_data} ->
+            abs(entry_height - height) <= 0.5
+          end)
+          |> Enum.min_by(fn {{_, _, entry_height}, _lms_data} ->
+            abs(entry_height - height)
+          end)
+
+        # Extract the LMS data from the closest entry
+        {_, %{l: l, m: m, s: s}} = closest_entry
+
+        [{"height", {l, m, s}}]
+    end
+  end
 end