stubbing out gravity model

doc/07_gravity.md

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+## Gravity Model 
+
+### CDB_Gravity()
+
+The Gravity Model is derived from newtons law of gravity and is used to estimate the degree of interaction between two places 
+
+#### Arguments 
+
+| Name | Type | Description | 
+|------|------|-------------|
+| t_id     | bigint[]    |     |
+| t_geom   | geometry[]  |     |
+| t_weight | numeric[]   |     |
+| s_id     | bigint[]    |     |
+| s_geom   | geometry[]  |     |
+| s_pop    | numeric[]   |     |
+| target   | bigint      |     |
+| radius   | integer     |     |
+| minval   | numeric     |     |
+
+
+#### Returns 
+
+| Column Name | Type | Description |
+|-------------|------|-------------|
+| the_geom  | Numeric | |
+| source_id | bigint  | |
+| target_id | bigint  | |
+| dist      | Numeric | |
+| n         | Numeric | |
+| hpop      | NUMERIC | |
+
+
+#### Example Usage
+
+```sql
+SELECT CDB_GRAVITY ();
+```
+
+

src/pg/sql/07_gravity.sql

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+CREATE OR REPLACE FUNCTION CDB_Gravity(
+    IN t_id bigint[],
+    IN t_geom geometry[],
+    IN t_weight numeric[],
+    IN s_id bigint[],
+    IN s_geom geometry[],
+    IN s_pop numeric[],
+    IN target bigint,
+    IN radius integer,
+    IN minval numeric DEFAULT -10e307
+    )
+RETURNS TABLE(
+    the_geom geometry,
+    source_id bigint,
+    target_id bigint,
+    dist numeric,
+    h numeric,
+    hpop numeric)  AS $$
+DECLARE
+    t_type text;
+    s_type text;
+    t_center geometry[];
+    s_center geometry[];
+BEGIN
+    t_type := GeometryType(t_geom[1]);
+    s_type := GeometryType(s_geom[1]);
+    IF t_type = 'POINT' THEN
+        t_center := t_geom;
+    ELSE
+        WITH tmp as (SELECT unnest(t_geom) as g) SELECT array_agg(ST_Centroid(g)) INTO t_center FROM tmp;
+    END IF;
+    IF s_type = 'POINT' THEN
+        s_center := s_geom;
+    ELSE
+        WITH tmp as (SELECT unnest(s_geom) as g) SELECT array_agg(ST_Centroid(g)) INTO s_center FROM tmp;
+    END IF;
+    RETURN QUERY
+        with target0 as(
+            SELECT unnest(t_center) as tc, unnest(t_weight) as tw, unnest(t_id) as td
+        ),
+        source0 as(
+            SELECT unnest(s_center) as sc, unnest(s_id) as sd, unnest (s_geom) as sg, unnest(s_pop) as sp
+        ),
+        prev0 as(
+            SELECT
+                source0.sg,
+                source0.sd as sourc_id,
+                coalesce(source0.sp,0) as sp,
+                target.td as targ_id,
+                coalesce(target.tw,0) as tw,
+                GREATEST(1.0,ST_Distance(geography(target.tc), geography(source0.sc)))::numeric as distance
+            FROM source0
+            CROSS JOIN LATERAL
+                (
+                SELECT
+                    *
+                FROM target0
+                    WHERE tw > minval
+                    AND ST_DWithin(geography(source0.sc), geography(tc), radius)
+                ) AS target
+        ),
+        deno as(
+            SELECT
+                sourc_id,
+                sum(tw/distance) as h_deno
+            FROM
+                prev0
+            GROUP BY sourc_id
+        )
+        SELECT
+            p.sg as the_geom,
+            p.sourc_id as source_id,
+            p.targ_id as target_id,
+            case when p.distance > 1 then p.distance else 0.0 end as dist,
+            100*(p.tw/p.distance)/d.h_deno as h,
+            p.sp*(p.tw/p.distance)/d.h_deno as hpop
+        FROM
+            prev0 p,
+            deno d
+        WHERE
+            p.targ_id = target AND
+            p.sourc_id = d.sourc_id;
+END;
+$$ language plpgsql;

src/pg/test/sql/07_gravity_test.sql

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+select * form CDB_Gravity()