So, technically in a SMPS it's not (only) the inductor resistance that gives the non-linearity. It's the output capacitor that makes the ringing together with the inductor. So if the inductor is large enough the portion of the wave form will be linear enough for the purpose of calculation.

Not the input or output filter cap, the switch capacitance.

But those too: the output ripple of a buck (or the input ripple of a boost, same thing) is a small segment of the (much slower) resonance between the switched inductor and the filter capacitor. It's reversed every half cycle so you see a small segment of this going up, then going down, forever.

So, the output ripple is approximated as a parabolic wave, i.e., the integral of a triangle wave (plus switching noise, passed in through the inductor's parasitic capacitance and the filter cap's ESL). Which is close enough to a sine wave not to care, so it's further approximated as that.

The other side (buck input / boost output) has 100% current ripple (i.e., current goes to full, then to zero), so is approximated as a square (at low current ripple %) or trapezoid wave. The voltage waveform, then, is a triangle wave, or a triangle slope one side and a bit of a curve the other side (a hybrid parabola + triangle wave).

The triangle segment of that, assumes a constant current source/load. This is unrealistic; more often, it's a resistance of some sort, in which case the triangle side is actually a small segment of an RC exponential wave. An example would be local ceramic bypass caps charged by electrolytics (through their ESR).

Well you have a point there. The inductor is rated at DC1=0.77A and DC2=1.2A. They don't mention which current which is but I remember seeing another datasheet where one of the two currents where induction drops to 80%, so it's probably not constant in the whole current range.

Well... presumably one of those is the DC (thermal) rating, and the other is the saturation.

I'd suggest shopping at places that, y'know... actually know what they're selling.

What is that, RS? Think I've seen crap like that there...

Tim