---
title: "feature-access"
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{feature-access}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

```{r, include = FALSE}
knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)
```


The gdalheaders API uses the following (pseudo-code) schemes for feature access. 

The different feature loops, see https://github.com/dis-organization/dirigible/issues/5


### ALL

Get all the feature thingys. 

```C++
  double   nFeature = gdalheaders::force_layer_feature_count(poLayer);
  List/Vector out (nFeature);

  double ii = 0;
  while( (poFeature = poLayer->GetNextFeature()) != NULL ) {
   out[ii] = poFeature-><something>; 
   OGRFeature::DestroyFeature(poFeature);
   ii++;
  }
```

### IJ

Get a subset of feature thingys i:j - their positional index. 

For `c(0, 0)` get the first feature, for `c(0, n - 1)` get all of them. For `c(0, -1)` that is a special case, there are no features. 

```C++

  ListVector out(ij[1] - ij[0] + 1);
  double cnt = 0;
  double ii = 0;
  while( (poFeature = poLayer->GetNextFeature()) != NULL ) {
    if (ii == ij[0] || (ii > ij[0] && ii <= ij[1])) {
      out[cnt] = poFeature-><something>; 
      cnt++;
    }
    ii++;
    OGRFeature::DestroyFeature(poFeature);
  }
```

### IA

Get a subset of feature things, *arbitrary i* (in order) - their positional index

```C++
  ListVector out(ia.length());
  double ii = 0;
  double cnt = 0;
  while( (poFeature = poLayer->GetNextFeature()) != NULL ) {
    if (ii == ia[cnt]) {
      out[cnt] = poFeature-><something>; 
      cnt++;
    }
    ii++;
    OGRFeature::DestroyFeature(poFeature);
  }
 ```

## FA

Get a subset of feature thingys *arbitrary FID* (order irrelevant) - their FID *unique names*

```C++
  List out(fa.length());

  for (double ii = 0; ii < fa.length(); ii++) {
    GIntBig feature_id = (GIntBig)fa[ii];
      poFeature = poLayer->GetFeature(feature_id);
    
```